EZY do overbook flights. We see it on our check in sheets for popular flights, usually during summer holidays.
I have personally booked the 159th and 160th seats on an a319 back from PMI. And knowing this we arrived very early to the terminal. They told us there was no system to sort out who got the seats other than first to check in, however this turned out to be too much for them.
These destinations are not bases for EZY, and therefore not so tightly governed. They sort out their own problems with varying degree of skill - which is why you won't be surprised to hear that whilst sitting in the completely full aircraft there was a poor couple wandering up and down the aisle trying to understand why there weren't any seats left.
The ground staff at PMI had let every passenger that turned up with a ticket get onboard the aircraft before realising.
It is hard to blame EZY for the actions of random Spanish ground agents that bumped you. It isn't part of our normal procedures. However EZY should take responsibility and therefore hand over the full compensation ASAP; you will find it a time consuming struggle to get anything out of any company in any industry these days.
Hopefully see you travelling with us again.