Could be lining paper under all that paint , even if it's not, this happens quite a often for various reasons,
might be damp behind or just poor adhesion of the lower coats , sometimes this also happen if you dont clean the dust off the wall , post sanding, prior to painting, after a couple of coats rolling the walls starts to lift the paint.

This may be of relevance for walls and presumably emulsion paint, but in my business, when spraying lacquer onto anything other than new wood this reaction known as "cissing " it often occurs it's as a result of the new coat reacting to something; wax, oil polish, etc on the old surface, wax is the worst, for what its worth once wood has been waxed, it's often bu..erd for any other type of finish. Surface preparation is everythig when trying to achieve a good finish. My guess is you'll never really bottom the problem unless you strip the surface of all the old crap which may have accumulated over years. The alternative may be to seal the problem in, but a good wash down and "key" the surface may solve the problem if it's not too serious.

Coat of watered down PVA, assuming there is no oil, grease or wax there.

A picture paints a thousand word