A certain percentage of passengers just don't show up for whatever reason. This percentage is fairly predictable, so if the airline sells more seats than there

A certain percentage of passengers just don't show up for whatever reason. This percentage is fairly predictable, so if the airline sells more seats than there are on the airplane, the extra tickets are pure profit. Because they are good at their statistics, usually, it isn't a problem, but every once in a while more people show up with tickets than there are seats. There are other reasons that this can happen. An aircraft change (due to maintenance issues, for example) might result in fewer seats being available. In this case, the airline needed to relocate some employees and seats weren't available. For the record, I believe they handled it very badly.