Can taking wallpaper off the wall cause crumbling?

This sounds like black mortar which is what was used to render, I would reccomend taking it all off and having it re plastered, not ideal if you want to save money but will save you in long term as this will undoubtedly have blown patches and will only get worse, sorry if it seems all negative but see this issue all the time

Hi removing wallpaper is unlikely to cause this reaction. You could damage wall by scraping to hard , however this sounds like the surface was unstable under the paper which could be result from damp. If you clean down wall with fungi cleaner and apply a seal such as zinnser guardz this will leave you a good surface to decorate on

It's probably condensation ( black) if it feels cold or even damp it needs some heating on it plus air circulation, after all that it could really do with looking at ( damp proof examination) as in the fine dust when you stripped the paper the plaster hasn't probably dried out properly an they've put the paper on without either siezeing the wall or sealing it ( it's called phosferent ) it's just when the plaster is drying , I hope that helps and makes it easier for you to sort the problem thanks