Wireless Keyboard Volume Down acting different
Most likely a change was made to the Fn key setting. The behavior you are seeing is "Show Desktop", but generally requires pressing Fn-F11 to activate. (Note on my keyboard Volume Down is F11. It may differ on yours, but the steps below don't chnage.)

To change this behavior:


Open System Preferences.
In the second section, click on Keyboard, then on the Keyboard tab if necessary.
Uncheck "Use all F1, F2, etc. keys as standard function keys."