I suppose so. Contract or not ND is a right to work state so as long as they follow the law they can terminate at will.
I just think they made it into a witch hunt and tried to force a resignation that he obviously doesn't want to make. Which by definition is constructive dismissal. And some if it occurred while the contract was in force and in public statements that appeared to be an attempt to embarrass him.
But that's why attorneys are available.