This stump shows several problems- my heart goes out to the family!
This is one of the consequences to poor workmanship- This tree didn't seem to have an adequate face cut. The back cut looked to be equal to or lower than the face, it was a miss matched cut for sure as the uphill side of the cut was at the same height as the face cut. There wasn't any hinge left he cut through it, so it lacked a steering mechanism. The " stump shot" was present but because the back cut was lower than the front lower edge of the face cut, it wasn't effective in steering, or preventing the tree from slipping off of the stump.
On the backside of the stump a cut can be seen that was way off of the target placement- it seemed shallow, but it may have affected the lean of this tree.
By reading the "log" you can see that the angled cut (face) was miss matched with the bottom of the face cut- there was a kerf cut that closed first, pressure was placed on the hinge, but then it was cut off, the log just slipped sideways off the stump, probably landing directly on the worker.