drywall or sheetrock have a thin paper/cardboard like surface on the outside when wet it can wrinkle

My dad and his friend painted my bedroom. After about 3 weeks and some hot weather there appears to be wrinkles/creases appearing on the walls - only a few. Can someone, an expert explain why this is happening. The walls were painted after being skimmed over. They used apparently watered down 10 to 1 emulsion as as undercoat and then 2 coats of coloured emulsion.

Can someone enlighten me please!