No idea... I don't think any of us are growers of exhibition veggies!

I seem to remember reading about this. I don't have the book anymore, but I think it was excess heat, and/or excess nitrogen.

Aha, remembered the title, at least: Onions and Other Vegetable Alliums (Crop Production Science in Horticulture series) by J. Brewster.

It's kind of a pricey book now, but well worth looking for in the library.