SPOILER ALERT
I was told this by a friend, so I dont' know if it's true or not.
Basically, there's this kid trying to do the ritual to summon the brotherhood, and then you show up. Needless to say, the kid mistakes a strapping young warrior like yourself for a member of the Black Hand, and so he offers to pay you in order to kill his orphonage matron (apparently she's some bitch who starves the kids and refuses to let them be adopted but really, if you're willing to join an assasin organization I'm sure you don't care about a just reason, right?). Well, being the misguided rascal that you are, you accept and kill the matron, then collect the payment.
Little do you know, however, the Brotherhood abducts you in your sleep the next time you go to bed and takes you to their lair. Then they present you with three people, and it's up to you which one you kill. The logic behind this is "You took our contract, a life for a life" so now you have to fulfill one for them. (Spoiler, the Kajiit pretty much tells you it's him) Well, kill whichever one you want, whether it's the mercenary who hasn't done anything wrong, the mother of 7 who abuses her children, or the dickish Kajiit, and then you're in due to your devotion to the brotherhood.