Any opinions on what may have caused this?

Looks like those are ball-ball pushrods. Too late but Checkout the Manley swedged end pushrods in 5/16”, 3/8” and 7/16” diameters with .080”, .120”, .135” and .165” thicknesses. Very likely available in the length you need.
http://www.Manley performance.com/dl/cat/pushrods.pdf
nope, ball/cup push rods. The cup end is the end that failed.