The official reason given is "he was fired because of the egregious nature of the findings in a misconduct probe".

Supposedly there are rumors of sexual misconduct but I've not seen any official statements or evidence to support those rumors.

Beau denies those rumors and says it's a smear campaign...which is what someone who was guilty and trying to hide it would probably say but also what someone who was the victim of a smear campaign would probably say. Looks like he has a lawsuit against Marvel in the works but who knows what will be made public.