You're right. I think there's more to this story than people are accepting. The fact that Pettine snatches up a guy he supposedly fired means he likely had nothing to do with the firing. Did it come from above? Perhaps, but it obviously didn't come from Pettine. Therefore, there was likely some meddling going on with the Pettine's defense, ironically the strongest unit on the Bills.
Could've Pettine felt resentful a coach he didn't see eye-to-eye with, especially one that should've been shuffling the deck on the offensive and ST side? Next we see Pettine leaving for the Browns of all teams? This is the Bills, I wouldn't be surprised if they made Pettine feel I welcomed and unappreciated.