I always thought the flowers are only good if you want seed -- and that the effort robs the (root) onion of growth potential. Congrats on your strong growth! (see my post for a sob-story... sniff...)snicker

Michele,
You're probably right that the onions are flowering due to heat stress. It happens more readily when they're planted from sets rather than seeds or transplants.
Make sure the rest of the plants are well watered and they will hopefully not all go to seed (flower).
The plants with the flowers won't make the largest bulbs and won't fall over when they're done like the others. They are better to eat fresh as they won't keep and will get pithy centers if left out too long.
If you live in an area with regular rain, don't break the tops off or water can get in there and start rotting the centers.
Hope this helps,
-Mark