Why are my Rocker Arms Breaking?

[72stepside4WD]

About 2 weeks ago I had a backfire (through the carb) problem that was the result of a broken rocker arm. My mechanic replaced the rocker arm and told me that he rarely sees this and thought that the rocker must have been defective. The truck ran great home from the shop and then sat for a week. 20 minutes into my next drive the same backfire noise started happening again. This time it turns out to be another broken rocker arm on a different cylinder. Both broken rocker arms happened on the same side of the engine. What could be causing this? THANKS FOR YOUR HELP!