Protocols? Oh, it's OK to over sell tickets and then bump customers so you can position crew?
Yes, sometimes it happens, but it doesn't usually take too much ingenuity to avoid getting into this awful situation.
Sorry I should have been clearer. Why does there ever need to be an involuntary removal when you're overbooked?
Because the airlines policy was to overbook. It appears here that the seats were needed for operational reasons. If you don't get enough volunteers you have to remove people who don't want to be removed. There are varying degrees of that, and in the worst case it might be necessary to deplane everybody and start again. Far better (and likely quicker in the circumstances) than ending up in this mess.