Sometimes when I close my Illustrator CS6 file it automatically saves without asking me if I want to

Of course, if I didn't want to save the file this creates a problem. It doesn't happen all the time, just ocassionally and I can't figure out why it does it. Any thoughts?

I'm using an iMac 2.5GHz Intel Core i5, Mac OSX 10.7.5, Illustrator CS6 v16.0.0