Breaker has different types according to its point of application. When breaker is equipped with metal stip for tripping, It's either thermal or magnetic, Most common nowadays is thermal-magnetic. Thermal tripping answers for overloading, and magnetic for short circuit. Most breakers in our home have no protection for human, it offers only to protect wires from overloading and short circuit so that fire will not be created whichever case may happen.
What was happen to your story was single line to ground fault, for a 20A breaker, instantaneous tripping is not possible, otherwise you have to wait for a long time before it would be able to clear the fault (depending on its characteristic curve). For you to clear this kind of fault, you have to use GFCI or ELCB breaker which can sense even milli amps and able to trip the breaker. Your 150A breaker was tripped because of the following reasons:
1. Your 150A breaker is equipped with ground fault protection or
2. its tripping curve intercept the prolonged sustained low level fault in the circuit.
I hope I have answer your question.
Charlie D.