So with roughly 21,000 miles,you just had the tires rotated? With an AWD or 4wd, they should be rotated at least every 8 - 10 thousand miles. But glad to hear with as many miles you have on yours (and sounds like good service from it)that you did have the struts/shocks replaced!..
Not so sure of climate temp change being the cause of this, but I personally know of folks having tire troubles when traveling to such warmer areas. Maybe enhanced by not having tires rotated sooner and already showing some of this wear before you left for "snowbirding".