This article explains it a bit (though it's not clear if he missed the flight "due to disciplinary reasons" or he was being flown home "due to disciplinary reasons" - seems likely that it was the latter):
Further reports now claim the crossing was deliberate.
An official operating on the condition of anonymity told the
Washington Post that "This was a deliberate decision on the part of the service member to cross."
The soldier is claimed to have missed a flight home he was scheduled to attend due to disciplinary reasons.
An official also told CBS News the alleged defector managed to join the border tour after somehow managing to return through airport security.
A witness told the broadcaster that the man in question ran away from the tour laughing as he did so.
The United Nations Command said Kim Jong-un's tyrannical North Korean regime has detained a United States soldier, named as Travis King, who holds the rank of private second class in the US Army, for crossing the Military Demarcation Line from South Korea without permission
www.mirror.co.uk