I think him killing his pops was the last straw. I think Tommy is tired of Ghost being sneaky and selfish, and him always having to kill people close to him as a result of Ghost. He realized this when they were viewing Kanan's dead body. Tommy said "K said it would either be him or me" or something to that effect. I believe Tommy was thinking, how long til its me on that table.
Truth, Ghost is the architect of all this