The backside of the Sierras is know for extreme turbulence especially in the spring. On that day the winds aloft were stronger than normal at lower altitudes too. Strong winds across 14,000'+ terrain = the worst turbulence I've ever experienced and it lasted 20 minutes, I've been a pilot since 1972. ATC was complaining that I was 700' off my VFR altitude. Heck, I felt lucky to still have two wings.
I can't help but wonder if he had a structure failure.
Good point because mountains create weather systems of their own regardless what the surrounding weather appears to be all year long!