d3track said:
I have a theory that the temp change could kick particles back up into suspension
It's not so much that as the warm up causes the yeast trub, which became nice and firm from the cold crash, to become less firm. That allows it to be sucked up into the siphon more easily.
That's probably a factor, but in my experience, I agree with d3track. I've noticed that if I allow a cold-crashed batch of beer to warm up, little CO2 bubbles will actually "explode" out of the yeast cake, launching yeast sediment and trub back up into suspension.
I believe this is because as you cold crash the beer, the yeast are still (albeit very slowly) munching on sugars and producing CO2. Since it takes several hours for the beer to chill enough to put the yeast to sleep, they work slower and slower as the beer cools, producing CO2 which
stays in solution (because the beer is getting cold).
Think of it this way. Your beer is sitting at 68° F. You move it into the fridge, which is at 34° F. A couple of hours later, the beer temperature has dropped to, say, 60° F. The yeast are getting sluggish, but still doing a small amount of fermenting, and producing CO2. Some of it bubbles out of solution, but however much of it could stay dissolved at 60° F does just that - it stays in solution (since the beer is only getting colder and colder, and thus able to hold more and more CO2).
At some point, the beer gets cold enough that the yeast just stop completely. But during the cooling, they've added some CO2 into solution in the beer. A very small amount, but a non-zero amount nonetheless. Some of it was added when the beer was colder than 60° F, and stayed in solution.
Now, you warm the beer back up. Once the beer warms above, say, 60° F, any CO2 that was added into solution by the (sluggish) yeast when the beer was cooling past 60° F is no longer soluable, and comes out of solution. It does this at "nucleation" sites, or particles onto which it can form. The best candidates for this are, you guessed it, the yeast and trub cells at the bottom of the fermenter. So you get little geysers of CO2 bubbles popping up out of your yeast cake, spraying particulates back up into the beer you worked so hard to clear.
The bottom line is, once you've cold-crashed a beer (with or without gelatin) to clarify it, then to preserve that clarity, you MUST rack it to another vessel (either another fermenter or a bottling bucket) before you allow it to warm back up. Otherwise, it'll just get murky again. Probably not as bad as it was before, but why let it muddy up at all? You've made it clear, so keep it that way.