Since no one else has said it, I will. You likely have some water intrusion somewhere, which has caused the roof to release.
If enough of the roof is loose that you are getting your bubble, your time is limited. Make plans for it to come loose during your next trip.
I had a friend with the same thing, and found most of his rubber roof hanging off the back of the trailer at a gas station. It wasn't pretty.
At the very least, I'd sink some screws through a 1x1 across the width of the roof to hold it all in place until you can get it repaired properly.
It likely needs a new rubber roof due to age, so plan on replacing it.