It seem more like a "I refuse to work with him so I'm leaving" type of situation than a situation where he spoke up and got fired because of it.
Reese was ticked at Berube for using Mason, right after surgery with no practice time, in a game in which Mason agreed to help the team out of a cap jam (that would have meant waiving and possibly losing Colaiacovo - which Hexy didn't want to do because he knew he was dealing Coburn and probably Grossmann if he didn't get hurt vs. Rangers).
Berube pulled Zepp for Mason after letting in two breakaway goals. It wasn't like Zepp was crapping the bed.
I can see Reese livid, telling Hexy that Berube is an idiot for risking Mason's health (again), and Hexy saying, 'well, if you don't want to work here, there's the door, here's your package, OK?' It was mutual in that Reese didn't want to stay, and Hexy didn't want him to.
At any rate, it was a stupid situation and all on Berube for his lack of any common sense. The sooner he's gone, the better.