I'm not sure what you have in mind by mechanics. What @Numidius describes a post or two below yours is the sort of thing I have in mind - as rolls fail or succeed the GM narrates (ie establishes and set out) consequences, which can include revelations of unwelcome truths. These are guided by the relevant system considerations (eg in Burning Wheel the GM should have regard to the intent of the failed task as well as the Beliefs and Instincts of the character; in AW the GM should make the character's lives interesting make Apocalypse World seem real, and remain faithful to the established fiction.I find this incredibly difficult to imagine play out simply on mechanics only because I'd say the GM would have likely already decided or at the very least thought of this before play and could use mechanics, whether successes or failures, to push play towards the desired reveal.
So how do mechanics in this instance help or change whether it was mechanics that revealed it or the DM via the fiction. To me it looks like mechanics are but an illusion of a prethought out idea. What am I missing?
So in these systems, the werewolf thing (or the "I am your father" thing) wouldn't come from nowhere. It would develop over a series of situations, maybe as a result of snowballing failures, maybe as a result of iterating failures and successes.
In the Burning Wheel game where I'm a player we went to the Tower of Evard because my knight PC's wizard offsider wanted to check it out (mechanically: she succeeded on a Great Masters-wise check to correctly recall the location of Evard's tower in the Pomarj; so the GM had to introduce it into the fiction). When we got there we fought a demon, which didn't go to well but did earn my PC a new infamous reputation in hell as an Intransigent Demon Foe. Then, checking out Evard's tower, a result of a check (the mechanical and full framing details now escape me) resulted in my PC finding old letters that seemed to imply that Evard, the demon summoner (it seems) is my mother's father. (My relationship with my mother and my Belief about my family are elements of my PC build.) That's an unhappy revelation - an unwelcome truth in AW terms. I don't know how much the GM anticipated something along those lines, but he can't have planned anything about Evard too far ahead given that Evard and his tower were only introduced as elements in the shared fiction because of a successful check made for an offsider whom I generally control and which on this occasion I declared and rolled.
This sort of thing would, in my view, be an instance of the sort of mechanics-drive-character-arc thaat @innerdude is talking about. (And using his holy grail system, Burning Wheel!)