It’s not clear from the article that the agency fired Baldwin. That’s a pretty unlikely scenario. Most agencies are delighted to handle a performer with Baldwin’s acting credentials and name recognition, no matter what kind of a beast he is in his private life. It more often happens that the big-name actor fires his agent. It’s possible that the agency criticized Baldwin or suggested an alternative way of handling things that Baldwin didn’t like and he blew up at them, just as he blew up at the child.
(This interpretation provided by a friend who has a movie production company.)