I asked Norm sempai the reason for the difference in footwork when doing static vs dynamic ushiro tekubitori waza. For example, whichever hand Nage offers forward, he could end up applying the technique on either the left or right side of Uke depending on whether he employs a static or dynamic (ki-no-nagare) technique. Both methods are applicable but depend on the circumstances.

Static

If Uke manages to get fully behind Nage, so that both wrists are held firmly behind his back then Nage would employ the static technique. This involves stepping back into Uke‘s mid-rift, to regain control of the grabbed arms. The stepping back motion allows Nage’s arms to be brought in front of his hara, and he is then able to step to the side with the forward leg and duck under Uke‘s arm to implement a technique.

Dynamic (Ki-no-Nagare)

Nage is able to blend with the attack such that the leading arm is the one being ducked under, which is the opposite of the static case. The entry is more direct so the technique comes on faster.