Why causes is to immediatley cut off and backfire?

Backfire (through the tailpipe) usually occurs when the engine is running to lean. Fuel not burned in the regular combustion cycle is expelled into the the exhaust system where it ignites from the heat of the exhaust system. If an engine kills while being driven, fuel with enter the exhaust system via open valves, and ignite. Your problem is going to be hard to diagnosis, as the fuel system will be operating properly until failure. Same goes for any possible ignition problems. You can easily check the pressure of the fuel pump to see if it is operating at the lower end of pressure tolerances. Personally, I suspect a heat related issue where a component heats up, shuts off, then operates again after the engine cools.