My first thought would be full size pupae/final stage bees, that were cleaned out due to mites, chilled brood, or some other cause that killed them.
Finding dead bees on the bottom, etc., is common for winter. But your days should allow for bees to naturally die outside the hive, to which is preferred and carried out by the dying bees. So I doubt it's bees dying on their own.
Having this happen could be for a number of reasons, some good, and some bad. But I would do a quick inspection the next time you are able, just to make sure all is well.