Nitromethane and methanol WILL burn in the normal atmosphere, and do not need to be confined under pressure to burn. The percentage of nitromethane used is strictly regulated by NHRA, (and it wasn't 85% early this year when we last ran NHRA Top Fuel, it was 90% or so) you actually buy the stuff pre-mixed from VP Fuels, the only legal supplier of nitromethane for NHRA. A close friend of mine, Scott Palmer, has a Top Fuel car that he runs mostly on the IHRA circuit, but we have run NHRA.
The explosions are caused when you "put a hole out", meaning it doesn't fire and burn the fuel. The resulting load of fuel becomes uncompressable in the cylinder (but the cylinder has a static compression ratio of between 9.5:1 and 11:1, plus the bower makes about 100 pounds of boost) and it "hydraulics" the engine, literally using the pressure exerted to split it open. Each cylinder makes 1000HP. So if you put a couple out, there's still 6000HP available to try to compress the fuel and hydraulic the engine. Nitromethane burns the best and most reliably when you have the engine loaded real hard. So if the clutch is set soft, the tires spin, or the tires rattle, when it unloads the engine it will likely put a hole or two out. Whether or not it explodes depends on how lucky, or unlucky you are. If you get lucky, it doesn't hydraulic, if you don't get lucky, it explodes after it hydraulics.