This has happened to many. Not sure if its the climate or humidity but It has happened to many in Florida as well as other places.
I have been watching mine but then again I live in Arizona with low humidity and not much rain. It does get very hot in the summer and no sagging on my rig. Knock wood.
Other have used good quality 3M spray adhesive but I'm sure others will give you more information.
Here is a link to the latest thread on ceiling sagging.
Ceiling headliner unglooing