Here is what I seen..
1. No one at United got mad.
2. They (crew) were not standby, they were a "must ride". (Meaning that they had to get to that destination to operate a flight while maintaining FAA regulations due to weather impacting operations)
3. Whenever you purchase a ticket, you are agreeing to abide by the passenger agreements and CFRs.
4. He wasn't randomly chosen, the computer system used goes by who paid the cheapest ticket, whether or not luggage was checked, status, boarding priority, etc.
5. The flight wasn't originally oversold until the inbound crew encountered a missed connect due to weather impacting operations and legality issues which is why these 4 inflight personnels had to get onto this particular flight to avoid a cancellation of the morning flight as this particular flight from Chicago only flies once at 3pm.
6. He was asked numerous times to leave the aircraft by United officials, he leaves, changes his mind then decides to run past the gate agents back to the aircraft. At this point, he is classified as non-compliant and a security issue which is why law enforcement was called. This is post 9/11. That's a federal offense, you don't run onto an aircraft after being removed. Period. Point. Blank. Once the law gets involved, it is no longer United. That's Chicago O'hare Int'l Airport and Chicago PD. They told him numerous times to exit, nicely, and he didn't comply so there you have it.