The path to atmosphere via the broken weld in the regeneration circuit allowed a parallel path of exhaust to flow in this circuit (plumbing path).  The most recent fill of CP5's dewar combined with the PID reaction to this fill resulted in excessive vapor pressure above the pump liquid level which was sufficient to raise the liquid level in the regeneration line above its highest elevation.  This established a siphon of liquid which spilled out through the broken weld.  This is the only scenario where the indicated LN2 level in the pump could be < over full and for LN2 to be spewing onto the ground (another missed clue was the fact that LN2 was not evident in the exhaust line as would be required for my initial theory of the overall system being "over full".  

Gerardo and I removed a section of plumbing from the regeneration circuit which allowed as to "cap" this circuit and prevent future incidents of the this type.  

NOTE: 
All of the ambient air vaporizers on the X-arm are broken in this same place and will need to either be capped or re-welded (i.e., fixed) at some point.