I've been playing a bunch of Fallout 3 lately to prepare for Fallout IV, and one thing I noticed is that when I shoot cars with my Chinese Assault Rifle that was modded in with mega damage, they explode in mushroom clouds and leave behind radiation. Why is this?
In Fallout lore, the futuristic cars that were driven in 2077 were based on the prototype Ford Nucleon, a car designed in the 1950s which would have been powered by a nuclear reactor instead of a combustion engine. These derelict cars remained in the world after the bombs hit. If you shoot them it somehow triggers the leftover fuel in their reactor engines and creates a small nuclear explosion which leaves behind radiation. You'll have to suspend your disbelief for this particular section of the lore since by our Universe's physical laws this makes no sense at all. Also, you don't have to use mods to detonate the cars - simply shooting at them enough will do it. This is why it's a very bad idea to take cover behind non-exploded cars during a firefight - they explode with similar force to a mini nuke, and even if they don't manage to kill you you will be left with very low health and multiple crippled limbs. You can tell if a car is close to exploding because it will catch on fire with a small non-nuclear explosion several seconds before it detonates. Shooting at it in the meantime will speed up the process.