The internet is a lawless wasteland.
I've always been single and hang out mostly in Singles, so most stories I've heard and known of people leaving/coming back has been due to personal entanglements.
For example (all based on true stories with names changed
1. Bill posts a lot on the site, but doesn't like the responses he's getting. He leaves and comes back to see if people will give him a more favorable reception under his "new" identity. This seems to be the most popular reason. People want a "fresh start" and "clean slate," hoping they'll be able better liked next time around and/or more people will listen to them.
2. Sara really likes John and believes he likes her, but she wants to "test" his loyalty. Maybe they move their communication offsite, and so she opens a new CC account and starts talking to John as someone "new," to see if he's flirting with other girls.
3. Jen is being harassed and stalked by an online admirer, but she doesn't want to leave the friends she has here on CC. So, she closes her account and comes back as another, hoping her stalker won't recognize her and will leave her alone.
4. I've also known of people who use accounts of the opposite gender, pretending to be a guy when they're a girl and vice versa, for varying reasons like the ones above.
Unfortunately, that old saying really is true in that you never really know who you're talking to online.