My bet would be over watering. It seems to be a lot harder to under water them - when my left over tomato starts are nearly dead from under watering the onions are just fine.

Another possibility would be if you have an unventilated greenhouse (even a plastic one in a window - if that is the case they don't need it anymore) and it is just getting way too hot at some point in the day - again, if you had other plants in the greenhouse they would probably be near death before you noticed problems with the onions.

Overcrowding could be a possibility too, but I would expect them to be stunted and die down from the tips before they just fell over. My onions do better when given room - it seems silly to take up more space with them when they appear to be doing just fine in a clump but they really do much better and it is easier to separate them when they need to be planted out. I know it can be hard, but unless you are planting acres there are some that just won't make the cut.

Hope this helps. Best of luck to you!

I had to add that the thread below is one of the best that I have seen for the whole process of growing onions from seed to harvest. The thread is about allium cepa var. cepa (sometimes called the common onion) not allium fistulousum (or welsh onion that is seen as more of a perennial) but the process and spacing should be about right to establish a perennial bed.

I am assuming for the following that you want a perennial onion: There is a LOT of confusion out there about bunching onions! Due to the nature of language (Americanized English) and changes in agriculture over time some seed companies seem to list all bunching onions as allium fistulosum - or just as bad, all varieties of onions as allium cepa. All varieties of allium fistulosum should be perennial and divide at the base in the appropriate climate. As I understand it - the reason for the confusion (aside from laziness) could be that at one time most green onions (or bunching onions, etc..) at one time were largely allium fistulosum, but as time went by producers for many reasons largely (or entirely) have switched to varieties of allium cepa var. cepa that have the culinary characteristics of what people used to buy a variety of allium fistulosum for - So in a strange way both the latin name and the common name have been muddled up pretty much (I assume) because not too many seem to care - even though it would be like selling tomato seeds as if they were potatoes or vice versa.

Here is a link that might be useful: http://forums.gardenweb.com/forums/load/allium/msg0310064812977.html