Travelling frequently on EZY I've noticed that when travelling alone and not requesting a seat, I'll usually get a middle seat - the worst, whether forward or aft seems to depend on how many other seats are already booked.. Therefore I always pay and select a window.
When travelling with one other person, we normally get middle and aisle, therefore I usually pay and book window and aisle, more often than not someone is put between us and we swap, but sometimes we are lucky, as on Friday.
I don't think balance is relevant here as EZY flights are usually about 95% full, and when they're not, as on a flight we took on Friday which was about 60%, the pax are pretty evenly distributed. The decisions are not made at the last minute at checkin as nearly all pax check in online.
I don't think they have a deliberate policy, as people have sometimes implied, of splitting parties travelling together to force them to pay for seat selection.