Bubbles resulting from localized loss of adhesion, and lifting of the paint film from the underlying surface.
Applying oil-based or alkyd paint over a damp or wet surface.
Moisture seeping into the home through the exterior walls (less likely with latex paint).
Exposure of latex paint film to high humidity or moisture shortly after paint has dried, especially if there was inadequate surface preparation.
If blisters do not go all the way down to the substrate: Remove blisters by scraping, and sanding, and repaint with a quality acrylic latex interior paint. If blisters go down to the substrate: Remove the source of moisture, if possible. Repair loose caulking; consider installing vents or exhaust fans. Remove blisters as above, remembering to prime before applying the top coat.