The Big Crunch theory is pretty simple to explain... Well, irony at it's best!
The Big Crunch is likely to happen because the universe consists of anti-matter and anti-energy. The Big Crunch is quiet literally a universal paperball... (If you know what I mean...) This happens if all anti-matter and anti-energy gives up and lets all of the gravity fall on each other. (If you know what I mean...) Basically, as I said, it becomes a very compressed, universal Paperball and that's the big bang once more... Normaly, quantum-physically this is the process of the universe, more of the circle of life in this case. It starts again and again and again... With absolutely no end. This is called the Big Bounce cycle. (If you know what they mean[isn't my picture])
It's hard to explain, but there are also many other similar theories out there that explain it more precisely...
IAstroViz