Was it definitely the Quick select tool, and not the Magic Wand? They live in the same place on the toolbar, and have the same shortcut after all. If it was the magic Wand, and Contiguous was not checked in the Options bar, that could do what you describe.
Thanks Trevor. It was both. After considering the issue, and using the tool in Elements on my laptop, I decided somehow the tool driver file had been corrupted, and did another install, which thankfully fixed the problem. I'm new to CS6 and was very surprised to have this kind of thing happen. I did a separate backup of my uncorrupted CS6 program files, and plan to do another backup this morning, just to be safe.
Fortunately, it's all good.
If this sort of thing happens again, you don't need to reinstall Photoshop. Either reset the problematic tool in the Options bar, or reset the Photoshop preferences file, by holding down Shift Alt Ctrl immediately after starting Photoshop. I don't know how to do that with Elements