Winter onions splitting

Does anybody know what causes this? The biggest ones have split at the base, there have been about 4 so far.  I suspect it might be the rain we had a lot of a few weeks ago. I am pulling the biggest ones to use as the onion seems fine, just split.   They were planted as sets last autumn, I'm not sure what variety they are as I buy them by the pound in the market, and they have three kinds, brown, white and red.