Any number of reasons from technical (broken seat) to behavioural (intoxicated passenger) and everything inbetween.
I am curious why security (police) were required for what should have been an operational matter. This situation (which hasn't been fully explained) superficiously seems to be a management failure, and by that I mean how the specific situation was managed. Any experienced pilot or crew member will tell You there are many ways to "skin a cat" and resolve a difficult situation. Dragging a passenger down the aisle by their heels wouldn't be a sensible resolution for most of them, as United are no doubt about to find out.
Sorry I should have been clearer. Why does there ever need to be an involuntary removal when you're overbooked?