Just made four easyjet flights in the last 3 days. (LGW-Krakow for along weekend and LGW-Geneva for the motor show) Three were SB; one was not. It is indeed linked to seat choice.
When you select a seat, you can pick price, location on the aircraft and Speedy Boarding or not.
If you do upwards of 10 flights (5 return trips) in a year, it might be worth buying their Easyjet Plus card that gives you seat choice and SB and airport fast track every trip.
I had that for one year when I was flying a lot in Europe, but then cancelled it after a year when it was no longer worth it - just buy the seat option on each flight.