Hi Jiml
Actually you are correct that cells have built-in vents, however, the vents can become clogged due to manufacturing defects also. I am not aware if the cells that we have talked about here had the vents blocked due to external contamination. Basically, when a cell has a blocked internal vent, the internal short circuit builds up heat and pressure and has no where to go and therefore the cell now acts like a rocket propulsion system. If an external contamination blocks the cell, its possible that it would not cause the jelly roll to be exposed from within the cell.
Most of the time, NiMH incidents are due to internal short circuits. These cells when internally shorted may not "explode" but can and will become very very hot up to 200F and more. I am not a chemist but the black soot is one of the chemicals used to create the energy and I would not want to ingest this either.