Maybe Ctrl+Win+D? This creates a new "virtual desktop". This slides everything to the left as you described. You can switch to different virtual desktops with Ctrl+Win+Left or Right. More info about the Virtual Desktop hotkeys here: https://blogs.windows.com/windowsexperience/2014/10/03/keyboard-shortcuts-in-the-windows-10-technical-preview/
I've seen this question before. Ctrl+Win+D can be blocked by using AutoHotKey. See my answer here:
http://www.tomshardware.com/forum/id-3683306/removal-win10-keyboard-shortcuts.html
There is no simple "turn off" option (that I know of). To block it, download AHK as described in the above link and install it. Open notepad and paste in these lines:
; Shortcut for Ctrl+Win+D
^#D::
; Do nothing for this key, just return. This essentially blocks it.
Return
Save the file as "BlockKeys.ahk" and double click on it to open it. The script will show up with an icon in your tray by the clock. You can close the script out by right clicking on the icon at anytime.
If that isn't the correct keys, without more info about the *exact* keys pressed or mouse movements it's kinda hard to tell what it is. Go into a game and keep pressing the keys that you think it might be until you figure out the exact key combo. This will help a lot towards figuring out how to disable it.