I wouldn't give it much thought unless they were my family or "real friends".
Facebook friends are often people you have no (real interaction)
Various scenarios for unfriending someone
I can easily see unfriending someone who is an ex or friends of an ex whom you feel has no business knowing what you are up to socially anymore. I can also see doing it if the person is constantly filling up your wall with countless postings you have no interest in seeing or reading about. Some people overshare!
They posted something you find inappropriate on your profile page.
Maybe some of "their friends" have been trolling Facebook and they reach out to you. Some people use social media sites like online dating sites.
I'm not a huge Facebook user and wouldn't loose any sleep if someone unfriended me. Odds are we're not "real friends" anyway.