Yes, there are quite a number of possibilities, including something as simple as maybe Cena's contract was up and Costello decided not to use him further, or maybe UN offered him more $$.
But don’t assume that just because he’s no longer with them that ABB brass “fired” him.
Most pro wrestlers nowadays are intelligent people with good business acumen. And if they’re not, they usually have an agent who is – especially the bigger names.
It's all business.