Cynical, as you can imagine, there's been a ton of discussion on the Texas boards about this. The biggest reason why Texas would want to do this is money.
The Big 12 TV deal is terrible. Texas received $12 million from the Big 12 TV deal last year. Indiana and Purdue received $22 million from the Big 10's. If Texas joined the Big 10, as by far the biggest college brand in the entire country, it stands to reason they'd receive more than Indiana and Purdue. It also provides the benefits of getting to recruit the Big 10 talent as well, not just Texas.
Its also a better fit academically. If you look at college rankings and traditions, the quality of Texas academics is much more on par with the higher quality education in the Big 10 than it is the academically-challenged Big 12.
I think the biggest obstacle to such a move will be the Texas government. They'd have to approve any UT switch, and they're not going to leave A&M out to dry. If UT bolts to the Big Ten, they're going to want A&M to be assured of being okay as well.
There's also rumors Texas is talking to the Pac-10. There's a lot of thought that if Mizzouri bolts for the Big 10, and Colorado leaves as well, the Big 12 is in trouble, as other schools will bolt too. If that happens, Texas needs a backup plan, and it could force Texas into joining either the Pac-10 or Big-10..