There could a million reasons. Some common ones are listed below. Before that, I just need to tell you, --- friend request --- is an option in facebook where you can click on another user's facebook page (and where ever the button 'Add as Friend') and add them as friend. It should not by any means be automatic. If it is, your ears should stick up!
Ignore, spam emails - telling you that you have sent a facebook friend request because you have not, and you know it.
Check, whether you are on "https"... facebook.com i.e. the address/URL should be https://facebook.com and not "http:" i.e. http://facebook.com
Other than that let hackers decide what 9999998 methods could be in sending friend requests.
Let's have a look on what Yahoo answers tell us.
answers.yahoo.com link to the question
"If you are receiving the notices at your email account, and they include a link to Facebook so you can have a look, then I would assume that it is a phishing scam that tries to take you to a fake Facebook login page to harvest your username and password."
"Can Facebook automatically send Friend requests without your knowledge?I have been getting email saying that such person wants to b friends in Facebook..."
Please people, take into account this isn't your email account (and you pretty much know that in your email account you do receive a lot of spam suggesting, you've countless friend requests from I don't know what site, so please be calm and don't respond. Click it as spam.)
More answers via Yahoo Answers.