I think the emulsion has become soft in the extremely humid conditions and has reacted.
I think the first frame has an arrow from the backing paper, although that could be from your picture.
I once had some 120 film in a cupboard that was unfortunately above a powerpoint, we had some friends stay for a weekend and they went through countless cups of coffee in our spare bedroom, the powerpoint powered an electric kettle!
The film was stuffed, I had difficulty after unrolling the leader onto the spare spool. When I closed the camera back I found I couldn't wind the film on without great force. I removed the film and found the emulsion had separated partially and was sticking to the backing paper.
Mick.