Some of the causes for this would include debris in the tank which would hold the float needle off it's seat and cause the fuel to drain into the engine or out the overflow tube onto the ground.The screen in the petcock should prevent the debris from entering the carb but it sometimes doesn't . The screen could have been removed or come loose at some point. Cures for this would be to remove and flush out the tank and reinstall it with fresh gas. It also could be caused by improper float level adjustment. Check to see if the gas cap is venting or not. If it's not venting, it could be causing pressure as the motor heats up and forcing gas out from the pressure. If you remove the cap when this overflow happens and the dripping stops soon after, then suspect the cap to be bad.