What causes a push rod to come loose in a 3.0 ford engine?

Under normal conditions it shouldn't. But if the vehicle was ran without oil or very low on oil or incorrect oil, often ran at very high rpms with high mileage and poor maintenance it would shorten the life of the engine greatly. Another possibility would be if the engine was rebuilt or repaired and the connecting rod cap bolts were not installed correctly or torqued properly or bearings not installed correctly that condition could happen. One more possibility, if you have a teenage male driver in the house that could also be the cause. :)