Even beer that is supposedly not carbonated will have some CO2 in solution from the fermentation. The hops will provide nucleation points for the gas to collect and come out of solution. That, plus the air trapped in the pellets, will cause the foaming you describe.
That makes sense. So all things considered, it seems to be a bunch of variables that coalesced. I'm surprised that I haven't heard of this on the pod casts or in the books when describing dry hopping, especially since I've always heard to keep your secondaries with as little head space as possible.