I agree with RainmanP - I think your chain was too loose. On a downhill, you would probably be backpedaling to keep your speed from getting too high. That would change the rotation between the rear cog and the chainring, which could lead to throwing the chain.

If the chainline is good, just get regular about checking your tension. You probably don't need chain tensioners, but they could be a good idea....

Me, every time that I notice my chain getting slack (trackstands are the best way to tell, IMHO- if I can move the pedals a half inch forward and back in a trackstand without moving - something is very wrong) I break out the wrenches. On a fix, the chain is the most important thing, and also the most maleable (stretching).

Imperfect chainlines usually just result in more noise....