Here are a couple of papers on different colloidal and flocculation properties of yeast.
http://onlinelibrary.wiley.com/doi/10.1021/bp00021a005/abstract
https://circle.ubc.ca/handle/2429/31517
Since yeasts do not swim about, they act as a particle. A colloid is a particle that is held in suspension because they are small enough that the molecular movement keeps them in suspension at least for a relatively speaking very long time. So the key here is that they act in a fluid as a...
Perhaps what happens in a cold crash is as the fluid is cooled the movement of the molecules slows as the liquid comes closer to what would be the freezing point. This reduction of kinetic energy might allow the larger particles of yeast being held in a colloidal suspension to fall out of...