Mr. Synellinden (why do I think I am repeating a dirty pun I don't understand?) makes excellent points.
Let me turn my speculation to the United chain of command:
It seems to me the fault is with the director of operations for United at O'Hare and maybe his boss -- the North American head of ops (or maybe the worldwide head of ops). He must have directed the gate operation people to get this crew on the flight, no matter what. The gate crew and the pilot-in-charge would not, it seems to me, carry out drastic steps without approval from above. The director of ops is toast -- it's a position where one has to exercise judgment, and he (or she) didn't.
For example, the ops director can call another airline for space, arrange for a charter (or limo), or cancel (via HQ) the Louisville flight. What was done is unthinkable -- and of course, the goons in the O'Hare security force compounded the problem.
Sage Grouse
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'When I got on the bus for my first road game at Duke, I saw that every player was carrying textbooks or laptops. I coached in the SEC for 25 years, and I had never seen that before, not even once.' - David Cutcliffe to Duke alumni in Washington, DC, June 2013