In Oblivion as we all know the character starts off in jail with a sadist jerk mouthing off at him/her.

Yet it never explains why the main character is in prison.

I mean even your character doesn't know why he/she was arrested. And when Uriel Septim is asked the same question I'm asking he says. (Maybe it was destiny)

So my only guess is that Uriel ordered the hero to be imprisoned solely for the sake that he/she'd be there to get the Amulet of Kings and start the quest. (Since Uriel foresaw him/her becoming the legendary hero)