I gather that owning a propane vehicle had its risks with respect to exploding unexpectedly 
? I guess this would be true of any vehicle with pressurized fuel, such as natural gas?
Yes and no. Pressurized fuels have to have better containment than, say, gasoline or diesel. If you have a vessel of gasoline that isn't sealed properly, you can get fumes from evaporation that can escape by diffusion, convection or atmospheric turbulence. With a compressed gas, of course, you have escape under pressure when there is a break in the vessel's integrity. Furthermore, propane in vapour or gas form, is heavier than air, so when it escapes its container, it seeks the lowest elevation. It flows downhill and pools, where it can't dissipate. In an enclosed structure, furnaces and utilities tend to be in basements. Imagine what would happen if a significant amount of propane flowed into a sewer drain and somehow got ignited. On a ship like a ferry, the lowest parts of the boat is usually the hottest parts.
On the other hand, natural gas (mostly methane) is lighter than air and will fill a room or enclosed structure from top to bottom.
Such things don't necessarily explode "unexpectedly". Vehicle fuel tanks react in different ways depending on what type of fuel it has. For example, I've seen vandals attempt to blow up a car by stuffing a rag into a gas filler neck and lighting it. The rag just burns itself out if it's been stuffed in tightly enough. If the rag is just loosely placed into the gas filler neck and lit, chances are there will be an instant, finite explosion that the perpetrator might not even escape from. The initial explosion will like consume all the oxygen in the area of the fuel tank, resulting in no further combustion till more oxygen arrives and another ignition source is found.
Propane tanks are a whole other animal. Any leak that ignites will make the tank get hotter and hotter and will eventually increase the internal pressures whereby more propane will vent from the relief valve. That will produce an even bigger flare, resulting in what is called a "bleve" (pronounced "blevy" - boiling liquid expanding vapour explosion). You have to witness a blevy in order to realize just how spectacular it really is. Let's just say that if it were a propane powered vehicle, you had better be at least a few hundred yards away when it goes.
Natural gas needs to develope some "concentration" before it explodes. If you just lit it as it comes out of a container, it would look like a gas stove. If it builds up in an enclosed environment and then ignited, it will explode with the energy that it contained in what has leaked out. The stuff still in the container won't necessarily contribute to the overall release of energy. Though such an explosion has been known to lift a house off its foundation or in severe cases, blow a house to bits, the energy is not quite as far reaching as a propane blevy. A natural gas powered car that leaks will have a strong explosion when ignited but the outcome won't be nearly as catastrophic as that of a propane vehicle. Not that I'd want to be near either when they blow, but if I had to choose, I'd pick the natural gas vehicle. In terms of "unexpected", I would smell the n. gas first as it will rise and have the added smell to help me detect it. Propane will stay at my feet.
BTW, propane is still used to power things like forklifts that are used indoors. Propane burns much cleaner so you don't get the same horrible exhaust fumes that a gas or diesel equipment would generate. Such machinery cannot be stored indoors. I believe it's against the law in most municipalities.
