Whhile I have absolutely no flying experience to back this up, I'm pretty sure a lot of it has to do with the aircraft's position relative to other parked aircraft and/or ground obstructions. For example, you're more likely to see a powered back-up (with thrusters) at the middle gates (say, C15-C25) where there's only tarmac behind, rather than at the end gates in C, where there are other aircraft parked behind it and/or buildings nearby.

The size of the "alley" behind the aircraft (and subsequent room to steer it) seems to matter a great deal.