No offense, but I think it's for attention (and they come back under a different account to see what peoples reactions are) or they want to remove traces of their identity off the Internet(we live in a day and age where employers and admissions offices do searches on you) or they feel the site is going downhill and won't get any better from then on.
I think the same thing. Once I get enough Xper for a VS gift card I'll stop logging on as often since most all the cool people left. However, I'll keep the account in case I feel like asking a question or if I get too poor to afford my own clothes and need more gift cards.
I do and I don't want to delete my account. My original friends here, for the millionth time have all left. I've been here for a long time now and I'm beyond burned out. I talk to less and less people, answer less questions, ask even fewer, and what, moderate on top of it all? If of when I do, guess what? I'll just do it. There won't be a warning, just as there wasn't the last time. Some people just don't have the time, or have just had it.