Too much reading is a possible culprit:
Observational studies have found that, in general, a person’s level of education — and thus the amount of reading they’ve done over their lifetime — is positively correlated with their risk of myopia. But the direction of causation could go either way: they could have read more growing up, becoming both more educated and myopic, or for various reasons people prone to myopia could be more likely to read more. Some studies have suggested that both are caused by socioeconomic factors.
For what it’s worth, several of the eye doctors I spoke with felt that increased computer use — both in school and in the workplace — has indisputably changed the development patterns of myopia, making it more common for people to become nearsighted in their twenties or thirties, rather than the conventional pattern of becoming myopic during childhood and stabilizing by adulthood.