Gases are more soluble in colder water, so when you chill beer (which is saturated with CO2 as a result of fermentation), more CO2 can stay in solution rather than leaving as bubbles. Hence bubbles stop forming and rising. It's the continual rise of tiny CO2 bubbles that keeps yeast cells and other particles suspended and that creates krausen during peak fermentation activity and a head on freshly poured beer. Take away the bubbles and the sediment drops out a lot faster than if you waited for the solution to fully degas first.
The cold also makes yeast dormant, stopping fermentation (though if yeast are still active, you might be cold crashing prematurely).