There could be a few reasons why it's doing this, here is a few of them: Most common is improper surface preparation, lack of primer, etc... When the paint is applied to a damp or dust covered surface from sanding, etc... Excessive moisture... When you apply oil-based or alkyd paint is painted over latex paint... When latex paint is being exposed to moisture in a short period of time after the paint has dried.

If the bubbling then peeled to bare drywall, you simply did not have a good bond. Such spots are often over the mudded seam areas, where sanding dust was not removed, or a good drywall sealer was not properly applied. Where popcorn is removed dry, often a chauky residue is over the whole area. Use of a good drywall sealer is imperative.

Cut out the bubbled areas, patch with a couple coats of drywall compound, sand and prime. Then finish coat with a quality paint suitable for baths with showers. Traditionally, higher sheen paints are used in showers because of their higher resistance to humidity.

Why was a hair dryer needed to dry that spot? If paint is not drying, then perhaps there is a gap in the insulation. Also, how is the fan working in that bathroom?

What kind drywall primer do you use