That's because you're probably watching clips posted by Chiefs fans and bootlickers of the end of the celebration when Worthy and a teammate were making like Santa and an elf. As soon as he caught it he lifted up his shirt as if to show a gun in his waistband, and that's what was flagged. I've actually seen some clips where the TD was immediately followed by the Santa celebration, with the gun reveal deleted.
The league doesn't like gun-related celebrations. They keep flagging them and telling players not to do it, and the players keep doing it. I wouldn't be surprised if the league moves to ejections and suspensions for this next year.