The short version is that Minneapolis man has, for some reason (and there’s no reason to assume it’s a valid reason), been placed on the “extra scrutiny” list by the TSA — but they somehow managed to forget to give him the third degree before boarding. So, obviously, the logical thing to do is screen him after the flight.
Fortunately, the passenger simply refused to comply and walked away, since the TSA has no power to detain you. (They did threaten to call the Denver police and have him arrested, which is remarkable in and of itself, and entirely aside from the fact that it’s a completely empty threat.) And, of course, the DHS and TSA won’t actually discuss why this man is on the extra scrutiny list, even with the man himself, so there’s no way to resolve the situation.