Not always weight and balance - I've had to move 8 pax to the front 2 rows of an otherwise full EI A320 for weight and balance. Could have been due to having all the bins in the back though.
I think it might just be EI's preferred method of seat allocation - filling it up from the back to the front.
It also gives the FA's a bit of breathing space - I'm sure they appreciate the little bit of seperation they get from their customers!