Spoiled yeast in fridge?

So I saved all of the yeast cake from my last batch of Oktoberfest. I'm usually really good about filtering the trub prior to fermentation so I just basically poured everything from the bottom of my carboy into a glass jar and threw it in the fridge. For the first week or two it looked great, started settling out so there was the nice creamy layer of yeast at the bottom and the clear brown liquid on top. A couple days ago, however, I noticed that the lid on the jar was dented upwards as though the inside was pressurized. I opened it up and lo and behold a woosh of air escaped, so the jar WAS pressurized.
"No big deal" I thought, I hadn't done anything but cold crash it, and it's a lager strain, so clearly it must have just been fermenting a bit more in the fridge. But last night I looked at it and there's a solid 2 inches of what I can only call Krausen on top of it. It's a thick, gross looking foam the exact same color as the yeast, sitting on top of everything in the jar. It smells okay in there, but I'm not sure what to do. Has anyone heard of this? Is it worth the risk?

Am I the first one this has ever happened to?

No

I collected the yeast cake(wyeast 2565) from the secondary (I rarely do a secondary) of BM's Helles Belles by adding 1.5 litres of sterile water to the fermenter and then swirling and bottling it in 2 x 1L mason jars. Couple of days later I noticed on of the jars had developed a krausen and the lid I had screw loosely shut was bowed outward.
First time I had experienced this. Incidentally I just chucked this lot of yeast (after pouring off the water & krausen) into a 2L starter and have it going at the moment.
I was surprised at the activity in basically sterile water in the fridge?

That's funny you did that, because literally not more than 5 minutes ago I finished making my starter... decided to throw this yeast in there and see what happens. if it smells funky or attenuates to 1.001 or something then I'll know there's an issue, but otherwise I guess it should be good?