Passengers on Sunday's now infamous United Airlines flight said they were told by the airline that a computer randomly selected the passenger who was forcibly re-accomodated by airport police.

Was a computer program really to blame?

Reached Tuesday by phone, United's media staff said they couldn't go into nitty-gritty details about how its passenger check-in and seat assignment software works. However, conversations with several industry insiders give a sense of how airline systems decide who gets bumped.

As with any algorithm, it's what the humans put into the code that's key. And it probably wasn't truly random.

Once a flight is set, an airline's customer service software kicks into action. These complex software packages, generally outsourced from either Hewlett Packard Enterprise or IBM, combine all major passenger service functions, including pricing, shopping, reservations, ticketing, check-in and seat assignment, said Brett Snyder, founder of the airline industry blog Crankyflier.com.