People get fired from companies they founded all the time. Once you give up that 51%, you are always at risk. Nobody is allowing some sort of "you have to employ me!!!" clause into a contract like that. I have seen plenty if people sell companies that thought they had something like that. It never worked out for them.
As soon as the new owners have no use for Cuban they will sideline him. That day might not be immediately though, and it won't likely be as ugly as it could have been as he is still a minority owner.