Apparently the straw that broke the camel's back was an incident backstage with Carmella.Apparently there were three main reasons for his release:
1. Going off script with the mini DBry
2. He no showed promo practice and went over time on his "I'm better even though I lost" promo, which apparently was so bad Vince also fired the writer who booked that
3. Apparently he was constantly drinking during the European tour, and Big Cass is the jackass kind of drunk
Apparently he caused a scene when he tried to have Carmella forgive him for whatever caused their breakup in front of the locker room. Carmella told him no and he got angry and grabbed her so she couldn't leave and a bunch of the male locker room had to get involved (the Usos and Rowan pointed out).
No idea how reliable these guys are granted.
Exclusive: Incident with Carmella was the main reason why Big Cass was released by WWE | Wrestling News