Originally Posted by oldbobcat

The sudden slackening of the taut span of the chain, caused by shifting to a smaller cog, can trigger a wave that can throw the chain off a chainring, especially in bumpy terrain. If there are no defects on the chainring or chain, everything points to insufficient chain tension. Find the shop manual for your rear derailleur and execute the derailleur manufacturer's instructions for chain length and b-screw tension, not the bike manufacturer's. Bike brands aren't infallible. For example, years ago, we had a line of mountain bikes from Marin that needed extracurricular work to make them shift right.