A and C,
I can only guess, but Friendly Dispatcher (above) has already done that. In my experience, seats can become unserviceable for many reasons - but normally down to passenger abuse. Defecation, urination, vomiting, cutting through the belt (I know, how does the knife get in there), sucking the belt (SA pax), children rubbing in mayo/ketchup, theft of the belt, mechanical failure of some part of the mechanism; that kind of thing. We carry spare seat bases, but that can only fix a limited number of seats and issues.
In your case, as I stress, it appears that your flight was not overbooked. But to find out exactly why you were not boarded will probably be beyond even the customer service people you have to deal with. But it's a moot point anyway - it costs a huge amount to easyJet to put you up for the night and pay the compensation, so there will have been a very good reason for it and I know that all avenues will have been explored at the time (just to save the money!)
Normally when this happens before boarding, it's the last person or people to check in or attempt to check in that get bumped. If the issue becomes apparent when people are on board, then volunteers are sought first (though it's a good decade since I have seen this actually happen).
At easyJet we really do our best. We have good kit and good people, and it all works nearly all the time. But occasionally in aviation things don't go to plan, and pax can suffer. This time it was you, but I can assure you that there was no mal-intent nor fraud, just something that went wrong that couldn't be fixed on the day.