What happened to my frozen yeast?

TIL that there are organisms which not only survive temperatures under 0 Celsius, but are able to actively live in that range, as opposed to just hibernating. Nevertheless, I doubt that baker's yeast falls under them - it is the kind of factoid one would expect to read in popular books on food science. Also, it is unlikely that something adapted for life under such conditions would be most vigorous at +35ish, as yeast is.

My guess is that it thawed in much more prosaic manner. You either had a power outage that you were not aware of, or the cumulative effect of years of self-defrosting cycles was sufficient to enable several generations of proliferation.

You describe it as being both liquid and stinky - this sounds like they produced stuff and they suffocated in it, just like in overfermented dough. It would have been mostly ethanol, but mixed with other waste compounds. A pure water-ethanol mixture created by yeast won't freeze (it shouldn't go above the alcohol content of wine), but it is not outlandish that, with enough other molecules swimming around, the whole mixture was not frozen solid at slightly below zero. So once their "civilization" had multiplied enough to drown in its own waste, it may have stayed liquid, even though it was in the freezer. Also, for such a long period of time, some enzymatic reactions might have happened even at those unlikely temperatures, leading to further decay.

Bottom line, it seems that the yeast had an opportunity to multiply (at above zero) and took it.