Hogan,
  What happened there is commonly referred to as "popping a wet one". This happens when the drill stem is being pulled out of the hole and the "driller" leaves the mud pumps engaged. This results in the mud being forced out through the "joint" that has been unscrewed. This mud generally has a high viscosity and is "thick" enough to put a pretty good wallop one you at low pressures. Best case scenario, you get really muddy. Worst case when the pump is engaged at full power, it can sure result in serious bodily injury or death. I learned really quick how to tell if the pump was still running or not from seeing this happen first hand.

Plus the hand was using a metal shield rather than the traditional "sock". When one does not use a metal shield correctly, it can become a projectile propelled by the pressure of the mud flow. Big piece of metal to the upper torso equals big problems.

jrw159 :-)