I think the 'pullback' started as a pre-holstering maneuver. The shooter is done with that round of shots and is about to reholster. Just before doing so, he pulls back, does a last quick scan, and holsters. In this case, it makes some sense. But I think some people took that shit to the next level because they thought it looked cool.

Who knows. That's how I thought it came about. If anyone ever sees me do it, call me out on it.