Buried within McTaggart's piece on Towles getting sent down is this nugget from Towles:
"I have to talk to my agent and see the options I have and go from there."
Will he ask for a trade? It makes sense. I can't remember an instance in which a player was sent down two levels. It's clear that Towles had one more chance, and it looked like he earned that chance coming out of Spring Training, but has now lost that chance. He didn't get a ton of ABs, and he - like everybody but Bourn and (sometimes) Keppinger - didn't do much offensively.
The long-term priority for the Astros at catcher is clearly Jason Castro, given that Cash was called up, and Quintero will suffice in the meantime. Towles had 47ABs this season, and hit .191/.235/.319. In 2009, Towles got 48ABs, and hit .188/.250/.354. My guess is that Mills and Wade had seen what they needed to see, didn't see any improvement, and made a move.
I can't imagine Towles coming back to the Astros. But I do hope he gets traded to a good situation, where the Astros are able to recoup something for him.