A condition of brood
Chilled brood is caused by brood losing heat, usually because the bees aren't covering it adequately. The larvae lose colour and eventually turn black.
Bees don't expand quicker than a rate at which they are able to sufficiently cover the brood, so chilling is unlikely to happen in a natural nest. Chilled brood is likely to be caused by the beekeeper trying to expand the colony too quickly, such as by brood spreading or stimulative feeding in the spring, when the bees are already stretched to their limits, or perhaps by some manipulation such as making increase without making sure there are enough bees to cover the brood.
Chilled brood is probably most likely to happen in the spring when late frosts will force the bees to go into cluster or semi cluster, so abandoning the brood on the periphery of the broodnest.
I have heard it said that inspecting colonies on cold days causes chilling, but this is not my experience. If the weather is cold an inspection will need to be quick. There is residual heat in the combs and I have never had a problem.
As with many things in beekeeping chilled brood is caused by carelessness. Make sure your colonies are densely populated with adult bees and you shouldn't have any problems.