Here's my theory.
If you were out of alignment before, just a little bit, and then rotated your tires then your tires were worn slightly unevenly. Then when rotated, your tires are now straight and true so not as much of the tire is touching. After a few (maybe a couple hundred?) miles they will wear to where they should be according to their current location.
That's just my idea. I just switched back to my summer tires and they slipped just as much as my 1 inch thinner All Seasons. After a couple hundred miles the tires started to stick. I know my alignment was off when I took the tires off, so this is my story, and I'm sticking with it.