That's your problem. Was he your starter throughout the year? Looking at his stats, I'd lean towards saying he wasn't just because of his stats, but I'm not sure what sort of playbook you're running. If you didn't start him, you'll fail the "Playing Time Promise" pretty much outright, and he'll transfer. Simple as that.
And I haven't had this problem in any dynasty I've ran. I've had some odd players want to go pro (my old Bama dynasty saw Richardson stay his senior year, and then Lacy go pro with him that next year), but never had problems with 10+ juniors or any sophomores wanting to go pro.