He was probably the one who called in the police.
Looking at similar flights from Chicago to Louisville, and having heard it was United Express, it was one of those little planes (under 60 seats) so removing 4 passengers (who were already seated) was a big deal.
An attorney on NBC affiliate last night said the airline would be right if they bumped you before boarding, but once seated the rules change in the passenger's favor.
I’ll bet that all rules end the same way.
“Or whatever the company or Captain want.”