I think I just found the answer to my question in my build thread about decking...Hydraulic expansion is my guess as well. I'm in SE Texas with a pool built in/after drought conditions-2012. My pool deck did the same thing after about 6 months at the union to the existing patio (even with re-bar tie-in). My PB came back and redid a 16-inch swath around the entire patio. Not an easy task, but we didn't feel the deck would correct itself. It's holding firm after 6 months.
Pragmatic, is there any way to prevent this? So this is a common occurrence for concrete decking in TX? I can see this being a major tripping hazard later. Thanks for posting this info.