What went wrong with this wall, and how do we fix it?
It looks like the skim coat released , this can happen for many reasons the most common is getting wet. On the ceiling it may have been caused from poor prep work when the remodel was done. I don't see evediance in that 1 section that the new coating stuck to the base surface at all. So making sure the surface is clean will be important, I used to always wash walls in kitchens with TSP, trisodiumphosphate, I think a safer type is available than what I used to use but it is a very strong cleaner that removes grease and other surface contamination prior to resurfacing or painting. I think this is what wrong but it could also be from being wet, or if the skim coat was two dry when applied. In any case remove any loose areas clean then recoat if moisture is not found, if moisture is found the leak needs to be fixed first.