This is a normal bird "thing". It's known as "whiffling":
Whiffling is a term used in ornithology to describe the behavior whereby a bird rapidily descends with a zig-zagging, side-slipping motion. Sometimes to whiffle, a bird flies briefly with its body turned upside down but with its neck and head twisted 180 degrees around in a normal position. ... This erratic motion resembles a falling leaf, and is used to avoid avian predators or may be used by geese (family Anatidae) to avoid a long, slow descent over an area where wildfowling is practised.
I've seen it quite often, usually (in the case of our pond) when geese are coming in over high trees and want to lose altitude rapidly without gaining (much) airspeed. Apparently eagles do it for play/fighting (notice how the bird that rolls inverted sticks his legs up with talons extended). Kinda fun to watch.
The Goose flies upside-down article in the Belfast Telegraph has a photo of a goose doing it: