From what I've read, I think that when you create a character from scratch it randomly chooses you "past decisions" for you. That or it makes an educated guess based on the traits you choose. A "ruthless" reputation for instance might be more prone to renegade choices. Though, to be honest I don't think they've full explained that side of things to date.
This contradicts what I've read - my understanding is that at certain points it will ask you what you would have done in certain situations.
I'm just going to carry on my Evil Bastard Shepard save.