Last week or so I did a search on when was the best time to harvest onions. And I wanted to know WHY. This is the best thing I found on the subject from the University of Minnesota Extension (http://www.extension.umn.edu/distribution/cropsystems/DC7060.html)...

"Optimum harvest from the standpoint of maximum storage life (before bulb sprouting), occurs while the onion foliage is still partially (30-40%) erect, and long before maximum yield is attained (when tops are completely down and dry). Since yields may increase 30-40% between the stage when tops begin to go down, and the leaves are fully down and dry, it is tempting to leave onions to cure in the field as long as possible. The optimum time for harvest therefore, must be a balance between highest yields and reduced bulb storage quality. Furthermore excessively field-drying onions increases the risk of bald onions in storage."

I read somewhere else (I can't find it now, of course) that you really want all the leaves to come down within a week or so of each other. Hybrid onions usually have their leaves die off faster than the open pollinated onions, which can take weeks to die back. I don't remember why you want your onion leaves to come down within a week, but it did effect the storage capacity of the onions. Maybe it could cause "bald" onions? I remember reading about that.....

Now, I have no idea how onions are grown in your zone or if the heat is what is causing your leaves to die. If you have good sized bulbs, maybe they are just mature and ready to be harvsted. I hope someone from you zone with more experience can help you out.

Good luck :o)
Kitty