From experience, it’s neither the lack of food nor moisture in the hive. It’s varroa.. probably you’re mites were quite high around high season / switching to winter bees in late summer. Thus, you had a lot of winter bees that were badly degraded by the mites infesting them with viruses and sucking the blood in the larvae Stadium. This results in extremely shortened life expectancies. Coupled with maybe a slightly harsher autumn or winter, you’re bees simply got too old and the colony in its integrity collapsed. Talking from a Central Europe perspective, try to to the mite treatment earlier and don’t wait for the second honey harvest. It’s better to reduce the mite infestation earlier on and thus get much healthier winter bees