Why am I accidentally changing the Project Bin icon to a multi-page symbol?

Running PSE9 under macOS Sierra.

I regularly create multi-layer collages starting with a background, then opening a file, touching it up, dragging it onto the background making it a layer, closing the file, and going to the next file.  Standard stuff.

Every so often, I do something accidentally which changes the file icon in the Project Bin to what looks like a multi-page symbol, and at that point I've lost control.  If I click that icon, I get pushed into a world I know nothing about, and I can't get back where I belong.

I would appreciate someone telling me what I'm doing accidentally so that I can stop doing it.  As a bonus, if you can give me a two-bit dissertation about what I'm getting into when that happens I would be a happy camper again.