Windows minimize to desktop

I know there must be a setting for this issue but don't know where to find it.
When I have 2 or more windows open such as mail and Safari and I sort of move the mouse horizontally the two windows minimize to the desktop, not the dock. I can click on either one on the desktop and it comes back to full screen. It's no big problem but I can't figure out what is causing it. Everything is up to date.
thanks for a reply...