I guess it would have something to do with "forbidden knowledge", but what exactly *is* "forbidden knowledge" in WH40K context?
In 40k terms, almost anything
Here's the list of Forbidden Lore skills from Dark Heresy.
The Black Library, Cults, Daemonolgy, Cults, Heresy, Inquisition, Archeotech, Mutants, Ordos, Adeptus Mechanicus, Psychers, Warp, Xenos
and from Black Crusade, a few more
Adeptus Astartes, Codex Astartes, The Horus Heresy and the Long War, Navigators, Pirates
The Imperium doesn't like its normal subjects knowing about anything they don't really need to know for their day to day lives.
Now, if you a Sister falling to Tzeentch, I'd suggest having her form part of the retinue of a charismatic Ordo Hereticus radical Inquisitor. http://wh40k.lexicanum.com/wiki/Recongregationism
This would have her separated from her Sisters who might be able to council her on the wisdom of certain choices. It would also expose her to a lot more of the universe than most would see.
Perhaps she has a run in with a Thousand Sons sorcerer who tells her of his actually meeting the Emperor, and that he was no God. In an effort to prove his 'lies' she delves deeper and deeper into forbidden tomes, where she perhaps finds out that what she thought was true is less .. black and white .. than shes been told.
BTW. Do Chaos gods reach out to their new recruits directly? As in, they manifest and give them powers on the spot?
Not usually. It's more likely to be a manifestation of ability or something.