There's some talk about Caldwell being the heir apparent for Summit at Tenn whenever she retires. I'm not sure what going to an SEC school does for that.
It doesn't make sense in terms of getting away from a dominant conference foe. Perhaps Tenn is more vulnerable in the SEC than Stanford is in the Pac-10, but she has to rebuild at LSU while she was getting it done at UCLA. It also doesn't make sense in terms of when the legends will retire and make an opportunity for the other schools: Pat is only 1 year older than Tara.
Caldwell is from Tenn. Maybe she just wanted to be closer to home and/or isn't a fan of the expensive/high tax/ridiculous traffic of living in LA.