This is a really interesting one and one which has come to light within PGMOL for focus.
Essentially, to paraphrase the guidance we've been given, the taker of the corner (or other restart for that matter) is permitted to leave the FOP as part of taking the kick (i.e normally they would re-enter as part of taking the kick); all other players should be on the field of play. By being off the field at the kick and then re-entering to take possession, the second player effectively re-enters without permission and interferes with play, which should result in an DFK and caution according to LOTG.
BY making sure the 2nd player in of the FOP, the officials are proactively preventing an offence that players are most likely unaware they are about to commit.