Could be a water thing. Onions are very deep feeders so nutrients and water needs to get the bottom of the pot. Onions will flower early and their leaves will dry if the plant is under stress. They would be happier to be grown in the ground where they can be grown further apart so the bulbs will be larger.

Thanks Jimmytheone. Sounds possible as I do grow them in containers. However, I do give them lots of water. Does this mean that I can now harvest or should I wait? I know onions can be left for a while.

I would stop watering them if most of them have dried out and bend the remaining stems over and let them dry out completely. Just cover them if rain is due. Turn them out and let the roots dry out. Use the onions that have the thick stems first and save the remaining ones for later.