I think Clowney is waiting till the 14th to screw with the recruiting services rankings. I mean they will pronounce someone number 1 and on down the line Wednesday but who ever he chooses could change it completely. That would be a funny enough reason to wait although he probably is still weighing his options and hasn't yet decided lol.
It could just be that he is a 17-year old kid who thinks it would be cool to announce on his birthday... sometimes we read too much into these kid's decisions... they are still kids.