The existing paint is flat so it is more than likely not an oil based. There are a couple of reasons why the old paint will peel off when dampened by the new paint.
1. When it was originally painted the texture(if textured) was not completely dry, the paint will dry first and not properly adhere to the texture.
2. The original paint was sprayed over a dusty ceiling.
3. The orininal paint (probably cheap) was applied without primer and the moister from the bathroom deteriorated the paint. I would guess this is your problem.
You did the right thing by removing the problem spot to the rock, to do the rest try useing a wet sponge on the existing old paint, let it set a few minutes(more than one application may be nesasary)and peel or scrape the old paint back to the rock as you did before useing a scraper or 6" taping knife. When done let it dry and lightly sand the ceiling with 150 grit paper being carefull not to damage the paper, dust off with a broom. You are ready to texture if desired, if you do, let it dry. When priming a bathroom I would suggest an oil based primer for moister barrier and after it dries you are ready to paint.
I have come across this problem with living room cielings with acoustic blown texture, if you are thinking this is a mess.