My guess is that either:
1) Brent decided he did not have the desire to play any longer
2) Brent has had another issue with alcohol or some controlled substance
Gonna guess the latter. The wording of Tim Macmahon's tweet made me think he did something.
The dude is an alcoholic. My gut is, he can't get it under control.
Besides, I don't think he was going to make the team with the way things are shaking out.
McClain, Crawford, Whaley, Hayden, and Bishop. Number crunch.