I was a student then. As I remember it, it was a fairly close battle between the two and Hayden Fry put both of them in at different times. It was Bohannon's first year as he transferred from a JUCO. I think it was just a situation where Bohannon eventually became QB #1. He was the better passer. It was not due to injury, at least as I recall.
Once upon a time in a land far away a coach played players when ever there was a chance to play them. Blow outs, to see who played better under real game time pressure. Not just anointing players to start indefinitely.