Sooner or later you will see bald brood in your hives and wonder what it indicates. It can be caused by hygenic behavior – bees uncapping diseased or mite infested larva for example. It can also be caused by moving frames around or changing the spacing. But in this excellent article bald brood is from wax moth larva moving under the capped brood, and causing the worker bees to uncap the tops – possibly in preparation for removing damaged brood.
While you are there you should check out the whole website - http://beeinformed.org - lots of good reliable information.