CNG, LPG, Propane and other gasses, which are compressed to liquid state can cause a BLEVE (boiling liquid expending vapor explosing) when the container fails or overheats.

When the tank is puctured, the gasses will rush out and cause a instand vapor cloud, which will expand and mix with the air. One spark and you get a big fireball and explosion. (unless the leak is really small).

The tank can also be overheated by fire, causing the contents to expend, which will cause the tank to fail. Other causes are overfilling and corrotion.

The chance that the tank will fail during a crash is very limited. There are many people driving on LPG, but very little is recorded of tank failures during crashes. If the tank does fail and causes a BLEVE, it will either be instandly or when the tank is on fire. Exceptions for pinhole leaks, which will cause a more controlled leak.