Somehow I have accidentally hidden my apps

I'm not sure how I did this I think I set an option somewhere but I can't find where. Only one window is visible at a time. It is like I have chosen "hide others" in the application switcher. As soon as I select an application all the previous one disappears so only the current one is visible.

I thought I did this by checking a box in OnyX as I was mucking around with that program but I have spent a couple of hours looking at it many times and everywhere else I can think of and I can't...

Somehow I have accidentally hidden my apps