It seems that a lot led to the firing of Big Cass.
With Big Cass being fired on Tuesday before the taping of Smackdown, there has been a lot reported about why he was fired. Fightful Wrestling reported on Big Cass angering WWE officials after he went against a directive on the May 1st episode of Smackdown. That was already known before he was fired but after he was fired it was reported that Big Cass had some behavioral issues on the WWE European tour
According to Wrestling Observer, Big Cass was punished for the incident on May 1st with the Daniel Bryan impersonator by being booked to lose twice via submission to Daniel Bryan. But, it was also reported he later upset some executives by blowing off a promo rehearsal and then went past the allotted time for that promo on the live show.
It seems that there were a combination of reasons, Big Cass was let go and Wrestling Observer reported that WWE left out well wishes in their press release because of how some felt about Big Cass.
It was also revealed in the report that Big Cass argued with Sami Zayn about geo-political issues regarding President Trump and Muslim immigrants but that is said to have not factored into his firing.
Big Cass has yet to make a public statement about his firing or his future in professional wrestling.