Why would they do that with a random traveler and not one of their own employees, though?

Perhaps because the dog would know the agents?

So get a volunteer from non-TSA employees who work at the airport in whatever capacity and clear it with their boss? I agree that this is a bit skeevy.

There are all sorts of alternatives. I'm just noting why they might not pick a local staff who has interacted with the dog.

I find the story interesting, but also it seems unlikely these guys are trying some real frame up job with a coat that their target is seen in the airport not wearing, and the security person carrying around. If anything it sounds like some guys goofing around.

The "aaaahh man!" reaction seems way too casual/jokey for a criminal setup that could have serious consequences for everyone involved. If I realized my scam target wasn't playing along, I'd probably nervously pretend that's completely fine and thank them for their time (something like that).