A person I know added me a few days ago. Today I got around to accepting it...only she says she didn't add me and that FB must be looking at mutual friends and when that number is high enough (5-6), they sometimes create an add. She said she had heard of this happening to a few friends of hers, too.
They've been doing those "People You May Know" things for a while now. She probably accidentally clicked on "send friend request" when trying to look at your profile.
That's what I thought but she vehemently denies adding me. Though, yes, I am open to the idea that upon looking at friends of friends, some unintentional clicks may have gone unnoticed.
Right after that, Facebook sent me an email. Subject: "Welcome back to Facebook."
Apparently I'm a reengaged monthly active user!