I've observed a number of poly relationships from the outside, as a friend of one or more of the participants. I've also been in a monogamous relationship for >20yr and I've lived on both coasts of the US in that time.

Generalizing wildly, "going poly" seems to be driven by one partner's selfishness and the other partner's desire to please. It has resulted in breakup of the original dyad in 100% of cases.