Not to defend Dennison too much because I don't think he did a great job, but he bent over backwards to try and adapt his offense to the completely unsuited personnel on the team. Play calling, blocking schemes, passing routes were all nothing like what he has done in the past. It was like oil and water.
At some point, I'm sure he decided that if he was going to get fired then it was going to be running his offense and not some bastardized, Frankenstein version of Greg Roman's so he went with Peterman.