What causes cracks in plaster?

If it is a plasterboard wall, the joints have not been taped properly, , if a stud wall, you have movement, boarded wall re_tape and skim, stud wall, check for movement, secure , re_skim, if its a new build, its settlement, wait 12 months and do the above

It is common to have drying out cracks after a full house being skimmed but if there are cracks appearing on the plasterboard joints then i would be quite concerned incase there are joints that haven't been scrim taped or if the wall's haven't been drylined properly which will also cause the joints to crack if there is movement in the boards.

Hi yes it is common to have some hairline cracks during the drying out process,this all depends on the thickness of the skim also if the boards have been dot and dabbed and scrimmed propely but if cracks are appearing in places other than joints then you have movement,really depends on what was being covered by the drylining and skim,or if it is a new stud wall will need to check if it is secured properly,plastering over existing lath plaster is not the best idea also as it could be loose plaster underneath,really need it check again as could be several things causing thus

Plaster on new plasterboard may crack because....the joints of the plasterboard have not been taped .... drying out shrinkage ...or movement