Mine has done that even here in Phoenix. It seems to be tied into the heated sterring wheel? Maybe it's outside temps, but I'm garage-parked 99% of the time and I think my truck hasn't experienced lower than 30 degrees in its lifetime.
Mirrors haven't self-heated in about a month and a half though. Chalk it up to Maverick Gremlins.
Pretty sure its tied to the outside temp. It will come on automatically when you start the car in the cold, but the heated steering wheel hasn't been turned on. When its warmer out, they don't automatically turn on. And like stated above, I think the set point is well above freezing for some odd reason