Why the yeast volcano?

I wish I had taken a picture, but I thought about it too late. Anyway, here's what happened. I had stored a good pitchable serving of trub from my last batch of stout. It settled out to about 6 ounces in a 12 oz jar. I had taken the trub from the bottom of my primary, and put it straight away into the bottom of my beer fridge. When I pulled it out a month later, I cracked the lid so that pressure would not build up as the jar warmed to room temp. I figured I would let it sit for a few hours before I pitched it into a batch of Nog that I was brewing. Funny thing was I left it alone for maybe two hours and when I came back almost the entire contents of the inside of the jar had somehow moved to the outside of the jar. This brown yeast mass had the consistency of chocolate mousse. Wierd!

Probably residual CO2 in the beer/yeast mix. As i warmed up, it started coming out of solution and took the yeast with it. Crazy, never heard of that with harvested yeast.

I thought that might be the case. However, when I harvested the yeast I thought the active fermentation was finished. Then I put the yeast immediatly into the fridge at about 40 degrees. I would not have thought that fermentation would continue but it seems that must be what happened....