What's going on here?

My Mac Mini is wired up to a 70" LCD screen to give brilliant graphics. It also means that I can view 3 or 4 screens at a time but I normally have just one screen covering the whole 70" because my seat is about 8 foot from the screen. But just lately the screen has been throwing a wobbler. It shows a perfect picture and then it will suddenly zoom out to show all of the available screens. If I click on the main image, that image goes back to full screen again for a few minutes before the cycle repeats.
Any thoughts.

You are toggling Exposé either via a magic trackpad, mouse or keyboard shortcut.

You'll need to check in Trackpad, Mouse and Keyboard preferences to see what all you have set up for Exposé to figure out what you're doing to trigger it. (mouse hot corner perhaps?)