Probably because he paid the lowest fare for that segment and had no frequent flyer status. When involuntarily denying boarding to a passenger, an airline in the US has to provide 4x the fare paid for that segment, up to a maximum of $1350. Some articles incorrectly reported this as "the computer randomly picked" vs "the computer picked".

The insidious part of it is that on more complex itineraries, the regional jet segments are priced rather low. I've flown from Montreal to Dallas-Fort Worth to Tokyo multiple times on American Airlines, and each time the ticket shows the cost of the business class YUL-DFW segment to be $232. If I was IDB'd from that segment (which also wouldn't happen to business class passengers without some major extenuating circumstances), all I would be legally required to be paid would be $232 * 4 = $928. I've seen economy fares on regional jet flights on complex itineraries of about $50, so the legal requirement to deny boarding to that passenger would be $200 cash.