Although, like any sane person, he might get frustrated with Dez for any dozens of reasons, I think Witten was solidly in Dez' corner.
I dont know about the rest.
I think Steven saw him as a bad investment,but that alone wasnt enough.
Linehan had major issues with him that only magnified when he threw the OC under the bus after the last game.
Sure, he made life more difficult in many ways for Dak, but I dont think Dak actually sabotaged him.
My guess is it was a combination of years of baby-sitting a grown man, the big contract followed by fairly JAG production, then Steven, Will Clay, and Linehan saying time to move on.
I wish him luck