Swingers can get bored of traditional swapping so they "switch"
Much like missionary sex can get boring after a while so can just swinging for some. People always like a variety in life and that applies to swingers too whether you think that's just greedy or not!
Many swingers will keep exploring, more couples, more adventure, more kink and finally into BDSM. They discover that it's not just the sex that is hot and also delve into the Dominant or Submissive roles. One of the more popular forms of what a swinging "switch" might be into is a Cuckold situation.
There are also situations where a Stag (& his Vixen) maybe into Dominant/Submissive type role play
Swingers tend not to get into the harder SM side of the BDSM they are more tuned into the Bondage and Discipline (or dominance) but do not often have urges for the more sadomasochism areas as they still very much want it to be about the sex and more into the "pleasure" than "the pain", where often the BDSM dynamics will focus more on the other fetishes which do not always revolve around the act of direct or traditional sex.