That's the cowboy/cowgirl phenomenon in action. A mono enters a poly tangle knowing full well that the partner is involved with others. The cowpoke, however, instead of wanting to work within the poly tangle, actually wants to try to pull the person away an into a mono relationship. It's rude and dysfunctional.
Does every mono getting involved with a poly tangle try to do that? No. Most folks seem to want to work within the parameters they accepted in entering a relationship. There are enough selfish, twisted people who have evil motives that crop up that it's a well-known phenomenon, though.
I suspect that my last amorata wasn't ever truly happy being in a tangle, though she wasn't actively trying to pull me away from my wife. She jumped from a relationship with me to a mono relationship in short order. I get the idea that she was just waiting for a mono relationship to pop up and was using me for support and companionship until that happened.
I'm uncertain as to how often that last circumstance would arise. I've not heard of it being widespread, though I doubt it would be apparent in most cases. I get inside info due to her and I having a daughter together, so my daughter (and her sisters) keep me informed.
When speaking of various forms of non-monogamy...it ain't poly if you're just fucking around.
While polyamory, open relationships, and swinging are all distinctly different approaches to non-monogamy, they are not mutually exlusive. Folks can, and some do, engage in more than one of them at a time--and it's all good.