Headliner glue has failed or foam backing is deteriorating. Depending upon how severe some just pin the area back up with some pins or screws but the right fix is replacement. There was a recent post from someone satisfied with work performed by a shop to replace the entire headliner.
I've exactly have the same issue. Air pockets bubbling in the headliner here and there. There are 3M adhesive spray cans. I'm sure there are youtube videos about how to best address the problem. I watch videos from Gears and Gasoline from time to time and I've seen the guys fix up beaters, but it's not in the title page of the videos so I can't direct you to a specific link. If I have to come across something, I'll get back to you. Vtuned garage does car restoration and body work. I'm sure his channel has videos somewhere in there. I was mulling the best way to take care of it myself, but just haven't gotten around to it yet.
Great, thanks all. The the foam is deteriorating. I've taped the sides for the time being, but seems like replacing the entire headliner will be the way to go. I'll do a search to see if I can find the recent thread on it