The unnamed witness, who was part of a tour group at the time, says the man "gave out a loud 'ha ha ha'" before he ran in between some buildings in the direction of the North Korean border.
"I thought it was a bad joke at first, but when he didn't come back, I realised it wasn't a joke, and then everybody reacted and things got crazy."
That part of the article is misleading. He was being sent back to the US for disciplinary issues, but he snuck off and got into the tour group, then bolted for the border. So I don't think it was a prank, he was trying to get out of whatever disciplinary action he was facing.