According to the article. The flight was apparently overbooked by 1. That was solved and pax were boarded. But then a need to get a crew to that destination now resulted in 4 more volunteers to been needed to deplane in order to make room for them. With not enough volunteers, pax including this guy were voluntold.
Sucks because this particular guy had nothing to do with anything or with UALs staffing issues down range. All he had done is buy a ticket, show up, board his flight, be calmly seated awaiting the flight he paid for, and no one would've remembered who he was under any normal circumstance.
But an apparently last minute need to send 4 crew needing a ride to the destination, now randomly became this guy's problem? That's UALs staffing problem, not his, or any other fare-paying pax (am assuming there no non-rev's or something to choose from). The concept of customer service needs a look here, as they shouldn't have to be paying the price, so to speak. A staffing need by the airline isn't an emergency that should get paying pax forcibly bumped. Volunteers? Sure. But after that, find another way, especially since the initial pax overbooking had apparently already been solved, and now was created again.