As said, aluminum will fail eventually. Steel can be run closer to its tensile limit (stretch is what breaks them, not power) and recover.
However: the calculation is complex. I'll leave out shape, metallurgy, heat treat, etc.
It's the number of cycles × some exponent of the highest RPM reached × spring pressure × some function of maximum incline angle × reciprocating load.
An aluminum rocker may last 50,000 miles turning 5,000 RPM, and explode if run to 8,000 even once.
Generally true for connecting rods also.

If you can't find a better answer to "what broke it", safest position: assume it's fatigue and replace all of them.
Once you get up on the learning curve you'll find that your vision works very well (shade tree FEA) in determining "there should be more metal here", and "that makes me nervous".