So what "caused" the lifter to fail?

I'm no engine builder however,

My theory is the push rod was flexing and pole vaulting the valve train causing the lifter to seperate from the camshaft. Possibly flat spotting the roller needles and or fracturing them when the roller was reintroduced to the camshaft leading to the destruction.

It would be interesting to see what the roller needles look like on the other 15 lifters.

Sorry about your broken toy Galen