There are a few reasons why a push rod will come out of it's socket in the rocker arm and or lifter.

1. Rocker arm nut came lose.
2. Rocker arm stud pulling out of head, or unscrewing.
3. Rocker arm broken in the area where the ball sits.
4. Valve sticking open due to tight valve guide.
5. Weak valve spring.

I'm sure there are a few more that I can't think of right now.