Trub dump causes explosive ferment?

About 5 days ago I pitched 6.5 gallons of porter wort 1.068 onto a Wyeast Northwest Ale cake in my 10 gallon conical and experienced fairly rapid fermentation.

Just yesterday morning after reading a gravity of 1024 I dumped about a quart of trub off the bottom only to come back later in the day to discover that there were some signs of explosive fermentation. In fact the fermenting wort actually pushed up the lid of my conical and I was left with a sticky web-like mass oozing out from underneath the lid.

Has anybody experienced this type of yeast activity more than half way through primary fermentation? The conical was only 65% full with wort when it started, is it common for this particular yeast let alone any ferment to have this much blowoff? Or is it possible that when I dumped the trub that an air bubble was formed that floated up through the wort?

So I guess another question is, is there a maximum yeast cake size that one would want to use to start another ferment? I mean, the yeast cake I had sitting there was probably around half a gallon...according to the measurements on the conical. Also, could infection be a possibility this late into primary?

Any thoughts would be appreciated. Thanks!

I'm thinking that dropping the trub stirred up some particulate matter in the conical and what you witnessed was actually a massive CO2 off-gassing and not real fermentation.

I experienced a similar thing with a two week old IPA. I dumped two ounces of Centennial pellets in and I almost had a mentos-in-the-pepsi reaction. Lots of foaming from all the CO2 coming out of solution from the hop particles, gives the gas in the liquid a nucleation point to form bubbles.

I tested the gravity again the past three days and it had dropped just a couple more points and stabilized over the past 24 hours.

Thanks for your insight, it helps me feel a bit more secure about this batch.