A and C, I have been bumped off a Lufthansa flight a while ago, my girlfriend was bumped off recently from a KLM flight, my sister last year same thing on a KLM flight. This stuff happens and I am absolutely sure, that if it is easyJet's fault, they WILL compensate you.
A while ago we had to offload a number of passengers because one of our flight attendants became ill and we could only operate back with reduced crew hence reduced passenger loads. So that's an other possibility.