Looking more into their design turns up all sorts of issues with nitrous oxide as a propellant. It's used all the time in dentist offices, so it's easy to misunderstand as an innocuous gas. At higher pressures and relatively normal temperatures it can go supercritical and become very, very volatile. At that point it's an even better solvent than supercritical CO2, and still a very strong oxidizer:

http://www.knightsarrow.com/rockets/scaled-composites-accide...

http://www.dtic.mil/dtic/tr/fulltext/u2/a489459.pdf

(In that Air Force research lab report there's mention of supercritical N20 causing a spontaneous explosion with coffee grounds.)