Resurrected onions

Bit of a gardening surprise yesterday (but, as I’m pretty new to this sport, what isn’t a surprise?).  I went out to pluck the few cantaloupes that hadn’t been attacked by bugs from my little plot, and eventually turned my attention to the onions.  I had read that onions can be harvested after their stalks dry up and flop over, which happen earlier this summer…I pulled a few of them for salads already.  I figured I’d pull up the rest…but then I noticed that they were no longer dried and shriveled (the stalks that is)…they were green and sturdy.  Did I miss my chance to harvest the onions?  Why did they suddenly spring back to life?  Comments welcome; in the meantime I’ll be doing some research…