THE L.A. TIMES has a big report on the Mojave / Scaled Composites explosion. Basically it’s a fairly standard industrial accident, made sexier for news purposes because it’s space-related:

Rutan said the suspected culprit, nitrous oxide, normally is “not considered a hazardous material.” Commonly called laughing gas, it is found in dental offices and is used by hot-rodders to boost the horsepower on their vehicles’ engines.

According to Rutan, company employees were examining the rate at which the propellant flows through an opening. He emphasized that the test, conducted at room temperatures, did not involve igniting the rocket motor or sparking any fire.

Probably something led to a spark in an unforeseen, and perhaps unforeseeable, way.