Trub Raising up to top of fermenter when dumping ....Weird!?

[Armando Soto]

Hello: I have a plastic clear fermenter, my process is to dump the settled trub at the end of the fermentation from a valve at the bottom. This clears some of the trub as it creates a "funnel effect" leaving some "trub wall" that then it settles back lowering the trub level. My goal is basically just to have the trub below the racking valve.
The weirdest thing happened to me with my last lager, at the end of the fermentation I opened the bottom valve to remove some trub up to the point I started getting clear beer. I usually let it settle a little more and do a second dump. But this time once I did the first one ALL the remaining trub suddenly raised up to the top of the fermenter. It just started floating, stayed there for about 3 mins and then slowly started to drop again to the bottom.
How is that possible? Why did that happen? Any comments will be appreciated..

 

[ListerH]

My first thought was CO2 coming out of solution and taking trub with it to the top... then holding it there until the beer "settled down" and then the trub dropped back again.

The other thought would be if the bottom of the vessel got warmer and a convection current lifted the trub?