Fired for "professional differences."
Read: Tomlin was pissed at poor STs play, he and Everest butted heads, Tomlin won.
Not that Everest is being pissy; he apparently wants to work in the league again and knows protocol.
Takeaway: Tomlin has to have been unhappy with STs for a while and probably had AE on a short leash; he's not afraid to pull the trigger when you
cross the line
go below the line (see Arians, "Fathead"