Dynamite is an absorbent material (usually diatomaceous earth) soaked in nitroglycerine. It can only be set off with a blasting cap. Dropping it, banging it, or going over a bump in the road will not (contrary to the driver's belief) make it explode. For that to happen, the dynamite has to have laid around for some time so that the nitroglycerine leaks (there being at least one infamous example of this in Western history). In short, the writer created a situation that would not have occurred in real life.