Why is my wall buckling and the plaster coming away In one area and how do I repair it?
Plaster doesn't stick well to wood in the long run.  On an old time plaster wall, the base coat was spread over lath boards so that it oozed out the other side and locked in the wall as a whole.  When that starts cracking, problems result.   This is too new of a house for that, so it's probably just some not-so-well thought out wall finishing.

The go-to product for sticking plaster to other things, including old layers of itself, is "Plaster Weld"  It's a pink thick latex that will bond to the substrate and let the new plaster stick to it.

I have an old quart, I've been nursing for years.  Seems it's not that easy to find these days.  Plaster Weld quart