carbonated yeast starter... what?!

Ok so I finally got into saving and reusing yeast, and tried to reuse yeast for the first time in my yeast starter, and had an interesting outcome. It seemed like it didn't want to go, but there was some action going on via the airlock, so I left it alone. Today I brewed, and grabbed my starter right when I was ready to pitch (why did I wait this long to check it?), gave it a swirl, and the liquid blew up like it was over-carbonated, bubbled all up. It smelled like crap too. I have smelled that on a bunk batch I had about a year ago, but Im curious how the hell this happened. Yes I sanitized, yes I did everything like I have done using fresh yeast and had no problem. Anyone have any ideas as to why this happened?

because the yeast generated a boatload of CO2, making a supersaturated solution, which you shook, releasing a lot of that CO2 in one go. i would be more worried if nothing happened! it's completely normal but avoidable.
with a yeast starter you want to facilitate gas exchange; that's why people use shaking platforms or stir plates, and don't airlock them, instead using foam stoppers or a loose piece of foil over the top. starters often smell disagreeable since they are usually grown above the temperature you would ferment the yeast at, generating a lot of the more 'fragrant' byproducts of yeast growth and then not metabolizing them, so depending on exactly how nasty we're talking, it's also to be expected.

I was just surprised because I never got that reaction, or smell, using fresh yeast. I cant put my nose on the smell, but it was bad. I tried a 10 split in growlers and half the batch had that smell. And why the foam stopper? I had one come with the kit, thought it was a cheap alternative to an airlock

TheMortReport said:

I was just surprised because I never got that reaction, or smell, using fresh yeast. I cant put my nose on the smell, but it was bad. I tried a 10 split in growlers and half the batch had that smell. And why the foam stopper? I had one come with the kit, thought it was a cheap alternative to an airlock

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The foam stopper is to let air in to the starter to help yeast grow healthy but still keep most bugs out.