I can reproduce this with 100% accuracy and STILL don't know what causes it... if that confuses you, imagine how I feel. What do I mean? Well. I can preform a series of forward 4's as sonya for the entire match in practice, let them hit me a bit to build up my xray meter (And they can hit me with anything... it doesnt matter) then xray to finish and it always works. Don't have to hold any buttons, don't have to be any specific variation or have any specific amount of health.....
AND YET. I've ALSO gotten off this brutality without doing any forward circles at all... so my theory currently is that it has multiple potential conditions.
I know that one of its conditions is definitely related to kicks though... which is probably why this brutality seems to happen most often in demolition, because my grenade combos involve a love more kicks then covert ops for example.