Yes, same problem. I have been told that the problem is caused by excess water that is trapped inside the onion-possibly entering through damaged leaves or getting wet while curing.

That being said, this is not a new problem for us. We try to be careful not to damage leaves, but it does happen. However, the number of onions that develop this problem are far greater than the number with possibly damaged leaves.

Also, we take great care to keep them dry after they are pulled.

Hopefully someone has a solution.

Happens to me every year, I also posted a question about it last year. Darned if I can find the thread even with an external search. I believe the answer was the same as for wolverine above, problems entering through damaged leaves. I remember not feeling very satisfied because it sounded as though there was nothing to do but plant more and cull more by checking periodically during storage.

Funny thing, I have a bit of a problem with onion maggots here but I rarely find them in actual onions. They are always in my leeks and garlics. Anyway, perhaps someone else will have more information.