There are vague rumors that something happened at Money in the Bank, that Cass was extremely upset and either did something or said something, possibly both, that pissed off someone important in the company. I haven't read anything stating about who that someone is, whether he or she is a wrestler or an executive, but the reports I've read state that it's not as random as it looks and that whatever happened at MITB was the final straw. Cass has been in trouble backstage before with the biggest issue, at least the biggest I've heard, is that he was a very vocal supporter of Donald Trump and was pretty confrontation if someone didn't share his views.
While I never like to hear about anyone losing his or her job, which is true even in this case, Cass is no big loss to WWE. Hardly anyone will even care by the end of the week.