It only happens with fists up + sprinting and is it not random. Hopefully this might help figuring out the cause, here is how to replicate:
Result: First jump will ALWAYS be the "slow vault" animation. The next and consecutive jumps will ALWAYS be the "jump" animation.
Good luck!
PS: The slow vault animation looks great when player is walking with fists down, but when in "attack mode" the transition from "fighting" pose to vault animation and back to fighting pose looks very robotic because it goes to "idle" pose (fists down/straight spine) before starting the actual vault animation. And it is anti-climatic due to the length of the animation, player is in "attack" mode after all, he shouldn't look or be completely helpless for this long. Maybe you guys should consider doing a different animation for when players has its fists up that is slightly faster/more aggressive, that would start with the "fighting" pose instead of "idle" pose. It would look and feel much smoother.