Not enough info. It's probably either too much or not enough rain.

We never rely on the rain, here! If just the older leaves are bent over, that's perfectly normal. If more than 1/3 of the leaves are bent over, I would suspect lack of water, or lack of potash. If the whole top is bent over, and the base of the plant is turning mushy or shriveled, certain root rots (there are several) may be attacking, especially if onions have been planted there more than one year in a row. Poor drainage can do the same thing. Fixing the drainage and solarizing the soil this summer should help with root rots, but an ounce of prevention, in terms of crop rotation, is worth a pound of cure.