Only way the pushrods can come out is excessive clearance. If nothing is bent / broken, you didn't over rev it, and the lash was set properly, you had valves hung open or you have weak or broken valve springs. Hung valves would be my guess. Run a leak-down or comp test on those holes. You my be able to see valve marks on the pistons with a light in the spark plug hole also. Did you run it hotter than normal?