There are multiple theories. Some say more people are now genetically susceptible to myopia. Others blame the increase on long hours spent in front of computer screens, possibly changing the shape of our eyeballs.
I myself A) use a computer about 12 hours a day (and have for quite some time), and B) am myopic (first having noticed in college, a few years after I began programming).
Perhaps anecdotal, I spoke to my ophthalmologist about this a decade ago and at that time he essentially said there is not enough evidence to show extended PC use causes myopia; however, he did say that myopia can sometimes be lessened in severity by spending more time each day focusing on distant objects (exercising the muscles in the eyes). To me, if one takes the contrapositive of that, it seems to imply excessive PC use might indeed lead to myopia.
At any rate, the (implicit) belief today seems to be that there is no causal relationship between increased myopia and increased PC use, but is that really the case? Or is it true there may be a genetic explanation for recent trends?