This was mentioned in the comments, but I wanted to get it up on our front page for readers who don't dig down there.  Per Moseley:

Williams left the Ducks' spring game after being hurt, and a medical examination apparently disclosed a previously unknown back issue that required he give up the sport. The former junior college all-American finished his UO career with two receptions, in a junior season marred by a hand injury.

Williams was a immense talent, and the Ducks were planning to see him on the field a lot this season with starter David Paulson.  That leaves the candidates to back up and team with Paulson as Colt Lyerla, Christian French, and Curtis White.  While that trio is long on talent (two five star and a four star recruit,) they have combined to never play a down of college football.

Best of luck to Brandon Wiliams.  Here's hoping he recovers from his injuries well.