It's the increases in energy density. You're packing lots and lots of energy into a small space. The challenge is to keep it stable. It's a similar problem to an explosive.

As people demand batteries that have much higher densities -- then it's going to push these boundaries... This wikipedia entry isn't a bad reference on it: http://en.wikipedia.org/wiki/Energy_density

Edit: .. and the rate at which you can tap that energy too.