Why does natural gas explode?

Last night there was a terrible gas line explosion in Philadelphia, captured on screen by a bystander (see video here).  The line was apparently being repaired when the explosion occurred. One worker was killed and three others are in critical condition.

Natural gas is comprised mostly of methane, CH4, with up to 20% other hydrocarbons such as ethane, C2H6. Methane is notorious for causing explosions, and I covered some of the physics of explosion in a previous post. Gases are flammable only under certain conditions.  The "lower explosive limit (LEL)" is the composition of a mixture (with oxygen in most cases) that contains the smallest amount of methane possible for combustion, and the "upper explosive limit (UEL)" contains the highest.  Below and above these limits there is either too little methane (LEL), or too little oxygen (above the UEL).  There is a quantitative difference between the flammability limit and the explosive limit, but for all practical purposes these terms can be used interchangeably.

There are two types of combustion: deflagration, when the combustion zone (where reactions are taking place) travels at a velocity less than the speed of sound in the unreacted mixture, and detonation, where the combusion zone travels at a velocity greater than the speed of sound in the unreacted mixture.  An explosion occurs when the container in which the reaction takes place bursts.  Control of the gas and vapor concentrations is a major issue in occupational safety and health.