The lucky split probably happened because his lumbar was bruised and he somehow stressed it, most likely by moving the joint.
The so called "random" fractures and dismemberments mostly happen because of the amount of bruise in the joint that gets fractured or dismembered.
Bruise means that red glow when your joints and bodyparts have been hit.
The deeper the glow, the more chance of breaking the joint.
Also, even though the it might not look like so, the bruise lasts unusually long.
In mod like Wushu, if your joint has been hit, I would avoid using it as much as possible, especially in comebacks.