Broken rocker arms two weekends in a row

Just to let everyone know, I replaced the two exhaust rocker arms that broke. Went Winterport Dragway and made 7 runs between time runs and eliminations and the car ran fine. I looked everything over at the end of the day and all looked well. I'm calling it problem solved. I'm still planning on replacing all the rocker arms over the winter. No apparent cause for the breakage other than perhaps the age of the rocker arms themselves.