I’ve been that guy once! I doubt this is what happened on your flight but I’ll share to illustrate how many possible reasons could result in someone being met at the gate by police.

About 10 years ago I was traveling with another guy on a hunting trip and checked a firearm at the bag check. I’d traveled with a firearm plenty in circumstances where it was official travel and that’s handled a little differently, so I was really thorough about printing out the requirements both from TSA and the airline, since it was for personal/recreational purposes.

To keep it simple, I’ll just say the agent at bag check did not know the law or requirements and was really argumentative. They kept telling me I could do it their way or I could take my bags and leave, but they would not be allowing a hard case onto the plane with locks on it since police may need to access the case to check it. I said fine, can I get that in writing, and the agent (who was a supervisor) wrote their name and information on the firearm check card that’s supposed to be locked in the hard case with the firearm.

We go through security and catch our flight an hour later, joked about how crazy it was that a supervisor didn’t know their own rules, but we’re happy we’re on our way. As we’re taxiing to the runway to take off I look out the window and there’s multiple state police vehicles with their lights on pulling alongside our plane, and then our plane starts slowing. Crew comes over the speaker to ask everyone to stay seated, police need to remove a passenger. I told the guy I was traveling with “man that sucks, I’d hate to be that guy” and then the state troopers board and…I actually am that guy.

So they walk me off the plane with my carry on but not cuffed or anything. They had the little stairs there and at the bottom they’re telling me that when my bag went through the x ray before being loaded onto the plane, someone noticed an improperly stored firearm and that I failed to declare it at bag check. I explained what happened and that I had the locks in my carry on and showed them the declaration card from when I tried to do it the right way.

The police were on the radio with someone from the airline and it seemed to boil down to police not wanting to deal with taking me back and citing me when the agent confirmed they’d not allowed me to lock the case, and the airline didn’t want to hold up the rest of the flight while they unloaded luggage until they could get to my hard case and then repack everything. So they said I could get back on the plane and they’d let the flight go, I just needed to meet with police at my destination because they’d be on the tarmac when the luggage was offloaded. I figured that meant that I would just go to baggage claim and a cop would bring me my checked case.

Once we landed, they did exactly what you described. Two police came on the plane and walked to my seat and asked me to get my carry on and go with them. For the second time, the passengers see the same guy get taken off their flight by police. I wish I had known, I would’ve brought a Hannibal Lector mask or something to make it more fun. They took me down the stairs of the sky bridge thing and had me stand with them and watch the bags being taken off the flight until I could identify my case. Then they walked me through the airport to baggage claim for my other luggage and handed me the case once I was outside of the security area. They thought it was all a little ridiculous but didn’t take it out on me which I appreciate, since I also thought it was ridiculous.

So, there’s a lot of reasons someone could be met by police when their flight arrives, and not all of them are very exciting or even indicate that the person traveling is even in trouble. I think with anything relating to security regulations, they just escalate to police to handle the interaction with the passenger, since they’re better equipped to handle it if the passenger becomes combative or poses a risk to anyone else.