Siphon started from my blowoff tube?

I had a yeast starter on a stir plate that made a really intense kraisen. I installed a blowoff tube and put the other end into a jar of sanitizer. Later, after it had fermented out, I put the starter, with blowoff tube still submerged in the jar of sanitizer, into the fridge to crash cool. I found out later that a siphon had started and sucked a bunch of sanitizer into my starter.

Can someone explain this to me, it seems that the pressure inside the starter would have to fall below atmospheric. Does this really happen?

The CO2 in your flask was at room temp, lets say the density was A. When you put the flask in the fridge, the density increases to B, which is more dense than A - i.e. more compact. Thus your flask equalizes its pressure differential by sucking in sanitizer. Don't be mad at it, it's just following the rules.

Cold liquids also can hold more dissolved gasses, creating lower pressure in your starter container and sucking the liquid up through the blowoff tube.

I believe it was due to thermal expansion/contraction. Water (or in this case beer) is less dense at warmer temperatures. When you moved it to the refrigerator, the density of the beer increased, resulting in contraction (shrinking volume). Your sanitizer solution also contracted, but since the volume was much lower than your starter, a suction effect was created due to the greater contraction (volume) on the starter side. Make sense?

Whatever caused it, this tells you not to use an airlock or blowoff tube in a starter.