Similar to comments from other posters, many people I know - and now their kids - experienced rapid deterioration in distance vision in pre-adolescence to adolescence. I've never looked this one one up, but they tell me the new and stronger prescription every year at this age is quite common.
As polarbear alluded to upthread, it has to do with a child's growth. When kids undergo growth spurts, the eyeball tends to grow as well. (And most myopia is directly related to the physical shape of the eyeball, which has a large genetic component.)
