It was Mitrovic letting our fans know he used to play at the Toon, he even alluded to it during his post match interview. Not defending fans but can't be surprised he got a rise from some of them.
He alluded to it when the interviewer pointed out that Sunderland fans were booing his interview and every time he touched the ball. He had a daft laugh and played the pantomime villain. You can't cry if a player gives a little back if you boo him every time he has the ball. Football fans are very precious souls at times.