There are some suggestions it's a screw job by Briatore.
He has stated repeatedly he's after getting Colapinto into the Alpine and is apparently still in discussion about it. It was Alpine's management that signed Doohan, not Briatore.
If they signed 3 drivers for next season it would probably come with fines and all the other craziness that happened with Van Guard, and they would probably end up with penalties to pay Doohan.
If they can get rid of him for his performance in a race, however, they wouldn't have those issues and could probably just pay him out (it has also been suggested that he is only "guaranteed" 5 races with an option for the rest of the season).