Facebook automatically sent out a friend request from my account. What the heck?

When I logged on to Facebook, I had a notification that somebody that I know had just joined Facebook. This in itself was odd, because I have never searched for this person on Facebook, nor do I know his e-mail address or even his last name; he would not know mine, either. I thought he might have found me through my mom, through whom he knows me, but she does not have such a notification or a friend request to or from him. We do not have any mutual friends, either, since he just joined.

Nevertheless, I clicked on the notification to see his page, and I saw that I had apparently sent this person a friend request, which I never authorized. What gives? Why was I even notified about his joining, and how did my account automatically send him a friend request? His this or anything similar happened to you?