If I had to take a guess, I would say he did it for security reasons. He spent the past four seasons here on the trading block, even though he was contributing well. Last year he even got dealt and brought back. Now, the Spurs are obviously playing the patient game with their veteran players to see how the Free Agent market pans out.
After a while, I'm sure anybody would get tired of feeling so dispensable. It's like trying to work hard for a job that could lay you off at any point. By going to Houston, Barry finally got to decide what HE wanted to do with his career.
When it comes down to it, the only person who really knows why Brent Barry opted out is Brent Barry.