occasional
How did you find out that the last airline you used started at the back and allocated seats as customers checked-in? Not everyone regards seats towards the back as lousy. I don't; I like to see what is going on in the cabin during a flight and a seat towards the back allows me to do that.
I don't think any of us is going to discover what method EZY uses to allocate unpaid for seats; at least not until lots of us have booked lots of flights with unpaid for seats and can perhaps get a feel for how they may be doing it.