I don't think the majority of teens are dealing with heavy academic workloads. If you're not in AP classes you're probably not doing much.

I think the heavy drop in teens versus preteens probably has a lot to do with smartphones. Like, I figure teens are much more likely than preteens to have their own smartphones. And preteens with smartphones probably have more parental regulation than teens who have them.

I also figure it's a problem that feeds into itself. They're reading less as preteens, so they don't develop their reading skills as well as they could, and this causes them to lose interest in reading as they get older. People aren't going to enjoy reading if it's too difficult for them.