The hypotheses you mention are not mutually exclusive. It's generally accepted that
- Walruses are sensitive disturbances like planes flying nearby etc., which have resulted in specific recommendations to people how to avoid disturbing walruses. This quote is for a different event (but from the same year, 2017); the general message on disturbances holds:
A survey Monday of a mile of coastline near the Inupiaq Eskimo village of Point Lay found 64 dead walruses, the U.S. Fish and Wildlife Service told The Associated Press.
Most of the animals were younger than a year old. The cause of death is not known, said agency spokeswoman Andrea Medeiros, but stampedes—set off when startled walruses rush to the sea, crushing smaller animals—are a likely suspect.
"Our thinking is, because of the age of the animals—they were young animals—it's likely that it was caused by a stampede, probably more likely than disease, given the age class," Medeiros said.
A polar bear, hunter, airplane or boat can cause a stampede. Alaska Native residents of Point Lay, who may legally hunt walrus for food, expressed concern after seeing an airplane flying near the herd and possibly circling.
"That certainly is a concern," Medeiros said. "That's not what we want people to be doing."
Fish and Wildlife Service guidelines instruct pilots of single-engine planes to stay at least a half-mile away from walruses on land or ice, and if closer, to fly above 2,000 feet (610 meters).
The guidelines call for helicopters and multi-engine aircraft to stay a mile away, or if closer, above 3,000 feet (915 meters). The agency warns that it is only guidance but creating a disturbance is a violation of federal law. [...]
Shaye Wolf, climate science director for the Center for Biological Diversity, who wrote the 2008 petition to list walruses as threatened or endangered species, said the Fish and Wildlife Service should review guidelines for protecting walruses.
"These animals are suffering a great deal of stress from climate change, and when they're pushed ashore, they should get very strong protections from disturbances," she said.
The ultimate threat to walruses is the rapid loss of sea ice due to climate disruption, she said, adding that rollbacks of climate change protections by the Trump administration will further endanger the animals.
- Their habitat has been reduced by retreating sea ice (strongly associated with global warming). That coupled with apparently a walrus population that has reached carrying capacity of their habitat, it's unsurprising to see them take more often to unsuitable environments, where higher mortality is observed. Since the event your inquire about appears to have happened in the Pacific, here's info on the walrus population there:
The latest research indicates that the Pacific walrus population in the Bering and Chukchi seas likely declined throughout the period from about 1980 to 2000 (MacCracken et al. 2014, Taylor and Udevitz 2015). The weight of evidence suggests that this population had actually approached the carrying capacity of their environment in the late 1970s - early 1980s, due to restrictions on subsistence harvests (Fay et al. 1989, 1997, Hills and Gilbert 1994). But, population models suggest a subsequent decline of approximately 50% (Taylor and Udevitz 2015), likely due to changes in vital rates associated with a population at or near carrying capacity. This decline has likely been exacerbated by declines in sea ice, which are associated with global climate change that are reducing the carrying capacity of the environment for walruses (Garlich-Miller et al. 2011, Taylor and Udevitz 2015). Hypothesized mechanisms include (1) the retreat of sea ice to a position over the deep Arctic Ocean basin, forcing walruses to use land-based haulouts where trampling events result in increased mortality to young animals (Jay and Fischbach 2008, Udevitz et al. 2012) and (2) the decline in sea ice reducing walruses' access to prey, which could affect adult female body condition, ultimately reducing calf survival and recruitment (Jay et al. 2011, Taylor and Udevitz 2015). While the use of land-based haulout areas is not novel for walruses, females with dependent young typically utilize sea ice for hauling out (Fay 1982), which allows them to avoid particularly large land-based haulouts where crowding and trampling events can result in large mortality events of dependent young (Fischbach et al. 2009). Unregulated subsistence harvests in the United States and subsistence and commercial harvests in the Russian Federation (commercial harvests ended in 1990) have contributed to declines of Pacific walruses in the past (Fay et al. 1989, Fay and Bowlby 1994). However, since 1992, harvest of this subspecies has been limited to subsistence takes by communities in Alaska and Chukotka (Garlich-Miller et al. 2006) and is currently not considered a threat to the population (USFWS 2011). However, a major remaining concern is the effects of declining sea ice on future energetics of females and young animals that must now make feeding trips from coastal haulouts to areas of high prey abundance (180 km one-way), rather than utilizing nearby ice edges for resting as they did in the past. Current research will hopefully soon shed light on this potential stressor. The status of the Pacific walrus stock in the Laptev Sea is currently unknown (Laidre et al. 2015).
Thanks to K Dog for pointing this out, there's newer piece in The Atlantic where more similar, fall events are discussed:
“Walruses have shown similar behavior on the U.S. coastline when space and ice were not an issue, and the reason is unknown,” says Lori Polasek from the University of Alaska Fairbanks. For example, in three successive years, from 1994 to 1996, dozens of male walruses fell to their death from cliffs in southwestern Alaska. But Kochnev and Lanfear argue that the incident captured in Our Planet is exceptional in both the height of the cliffs and the number of walruses that plummeted and died—hundreds as opposed to dozens.
The reason for the falls might be complicated, but it’s clear that climate change is affecting the walruses. “We do believe that haul-outs have increased in size due to the loss of sea ice—in part, due to females and their calves moving to land during summer,” says Nicole Misarti from the University of Alaska Fairbanks.
In this specific incident, there have been allegations in the mass media (mostly from the right side of the political spectrum e.g. in The Australian) that the crew of Our Planet actually spooked the walruses, which the crew denies doing; the Australian quotes them as
Attention now centres on whether the film crew had blocked the walruses’ exit path after they had been spooked by drones used in filming.
Documentary-maker Sophie Lanfear defended the film crew’s actions.
“When approaching the walruses, we made sure we were downwind of them and that we could not be seen,’’ she said.
“We only stood up when it was safe to do so and when we weren’t at risk of scaring any walruses.’’
The filmmakers used a walrus behaviour expert supplied by WWF.
“Fundamentally, the reason walruses used this haul-out location is because of a lack of sea ice in the region, meaning they are coming ashore more frequently than they did in the past,” Ms Lanfear told British media.
The crew has a longer page listing the precautions they took. I haven't seen wildlife experts saying that those precautions would have been insufficient, although I admit I haven't read the voluminous coverage that this incident seems to get in a certain part of the political spectrum (Breitbart etc.)