I suspect it has much to do with busy fleet rotations in and out of the various servicing routines. The AB6 and 734 fleets do both dom and intl, and its probably easier to offload pax at a bay, service, and tow to next gate (or board pax) than to always go to a gate, and tow to servicing or another gate.

There is probably an element of the 'ways things were done' at Don Mueang which has yet to be shaken out of the system.