Maybe the first lobe to go was the one with the most mechanical interference in the valvetrain, it just took the others a little longer to self-destruct. I'm just guessing here, but that's my best guess.

I suspect that this is not a case of a soft cam from the manufacturer, or you'd have a problem with both banks. I'd say the driver's side head was set-up differently from the git-go or maybe they aren't the same casting numbers and therefore one head has less tolerance for the parts you used to build the motor. Again, just long-distance guessing.