My theory is that Barthel had two mistakes working against him.
1: He didn't evaluate his room properly. When Emmett Johnson, who you had at 4th string, comes in and outplays the guy you had near the top of the depth chart (Grant). And that top guy cost you a game (Minnesota), then maybe you aren't a great evaluator of talent. And his nonsensical usage of his room in the rotation emphasizes that you don't know what situation is best for your guys.
2: He lost the only RB recruit we had. (Lacy)
This one isn't so bad because it happens when you go after a top talent, but he didn't have a back up plan ready to rock afterwards. And we know Rhule loves contingency plans and probably doesn't like it when someone on his staff doesn't have one.
Bonus Theory: Was Barthel involved with that staffer whose hijinks made it so that he had to be let go a few weeks ago for being a perv?