Sometimes I just don't understand yeast

I brewed an oatmeal stout 1.072 OG. Racked it onto a yeast cake, S-04 from a previous brew that had 1.080 OG and 1.023 FG. Started quickly and stopped suddenly at 1.033 and stayed that way for a week or so. I brewed another IPA with S-05. I racked the stout onto the S-05 cake and within 6 hours the krausen was about 3 inches high and the yeast are working again.

Before I racked it I tried stirring and bringing the temp up to 70F and no activity.

Now, what actually caused the stout to start fermenting again?
The different yeast? The big move? Luck?

I tell you what, let's work together: if you can figure out women, I'll try to figure out yeast.

Bartman said:

I brewed an oatmeal stout 1.072 OG. Racked it onto a yeast cake, S-04 from a previous brew that had 1.080 OG

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I wouldn't reuse yeast that has seen > 1.050 OG. Anything above that is technically toxic to the cells. My thoughts are those yeast were incredibly stressed and just didn't want to ferment anything else.

PseudoChef said:

I wouldn't reuse yeast that has seen > 1.050 OG. Anything above that is technically toxic to the cells. My thoughts are those yeast were incredibly stressed and just didn't want to ferment anything else.

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I hadn't heard this before, but it seems reasonable to me. That's why I don't pitch on an old cake/wash/otherwise reuse yeast that had brewed an actual beer - I take and store about a pint of my starter and use that to build my next starter. I repeat and I can get about 5 uses from a single pkg of liquid yeast. No need to wash the yeast, just chill and decant the wort off the yeast on. My experience is that these subsequent starters start at least as fast as the original.

You overpitched dramatically. That causes the excess yeast cells to compete for available nutrients and the result is way more yeast than you need, all of them unhealthy.