Why did Facebook notify me of a non-friend's birthday?

[Jenna]

I have the Facebook app for iPhones. Sometimes it will randomly remind of a friend's birthday with a notification message like "Celebrate with ______ today. Write a birthday wish on his/her timeline." However, one day, it notified me of someone who I'm not even friends with (we do have mutual friends though). Why?
I do not have a friend request from the person. We have less than 5 mutual friends, but I do know them in real life, and we are from the same area.

[abraXus]

do you have a friend request pending from that person?

[Kait]

You must've added them at some point, & either it's still pending or they declined & automatically you follow them. So, the only way is you happened to be following that person.