And did he have this problem at USC? It seems like he might have, but looked better his final season. I don't really know. Found this in an LA Times article:
But Williams is the nation’s most efficient passer, especially after improving his accuracy on passes in the intermediate and deep range. Williams has completed 81.8% of his passes between 10 and 19 yards this season, according to Pro Football Focus, a jump from 55.7% last year. His deep throws connect 57.4% of the time compared to last season’s mark of 41.8%.