[Foster]

It's really hard to say exactly. It sounds like you are designing and building your own system. So without seeing the schematics of what you've build it's hard to do remote diagnostics.

Things that can easily burn up parts of the PIC chip are overloads on the ports. The output ports to the HBridges can easily be over-driven (put out more current) than they are rated for. This will cause just that port to fail.

Your HBridge is also prone to excessive current flow which may damage the control parts or one leg of the bridge. It's also possible that back EMF (caused when your DC motor becomes a DC generator) feeds into the HBridge backwards and destroys the drive transistors.

Off the top of my head that's the two places I'd look (after low battery and loose connections)

Good luck!