Onion rotting question

[Compostman]

I planted onions for the first time this Spring. They grew very fast and I harvested them when the leaves began to turn brown and the onions were baseball, or larger, size. I let them dry some in the attic for a few days then put them in a dry basement. Even while in the attic, some of them began to rot in the center. Any idea why? Note: Although this is my first onion crop, I've been very successful with garlic and shallots.

[farmerdill]