A recent discussion about what exactly Hedi Slimane is doing that warrants all of his models signing non-disclosure agreements prior to fittings/shows resulted in a partial answer: Maybe the YSL-turned-Saint Laurent legal team is just doing its job to prevent collection-related information and maybe more importantly, images, from getting out there before show time. It makes sense. While we would all secretly love to think that Slimane is a wild and crazy artist, who behaves outlandishly and thus, needs to be protected from gossiping models exposing him to the world, that may not be the case or better yet, that's probably not the case in its entirety.
First of all, the employment of non-disclosure agreements (NDAs) in fashion isn't completely unheard of. In February 2011, for his first show back in the world of womenswear, Tom Ford asked show-goers to sign NDAs, preventing them from speaking about the collection or taking photos. (Side note: attendees at Slimane's first show for YSL were not required to sign anything). Ex-Balenciaga creative director, Nicolas Ghesquière signed a non-disclosure agreement with the house, which prompted Kering to file suit against him. But having models sign NDAs to walk in your regular runway shows? Not as common.
Having said that, since Slimane came on board at YSL in March 2012, the Saint Laurent runway shows have been some of the most anticipated each season, and so, it is only expected that the house would want to keep all info to themselves prior to show time. This includes the looks, the styling, and even the models that have been confirmed for the show. Slimane is known for casting an array of relatively unknown models as exclusives (especially for his mens shows) and keeping them a secret until they hit the runway. That is one very likely reason for the NDA.
Further, according to one of Slimane's October 2012 exclusives, Louise Parker, who spoke out (as much as she legally could): "We had to sign a confidentiality contract about everything. We weren't allowed to take photographs or tweet or Instagram while we were at work." So, an NDA to prevent to leaking of looks (especially in Louise's case, as leading up to the show, she was "trying on the clothes so they could figure out which skirt goes with which blouse, and stuff like that ... working every day, shooting looks and doing fittings") - another plausible reason for the NDA.
Another potential reason (and the one Cathy Horyn would likely want us to believe): the models spend a lot of time with Hedi leading up to the show and his behavior (which has been labeled as "terrible", "childlike", "narcissistic" and "self-important" - guess which publications said what) needs to be kept under wraps. According to Parker, "All of our meals were eaten together, and we got to talk to Hedi a lot — he took the time to get to know the models." Another model, said similar things: "All the boys work with Hedi very closely ... Hedi takes a lot of time fitting each boy because he likes to really get to know them."
While it seems that Slimane may be employing NDAs, in part, to keep what the press has largely characterized as erratic and bratty behavior from being exposed, models who have worked with Slimane claim otherwise. Louis says of Slimane: "I was expecting him to be really high-stress, or really nervous, but he was always very consistent with his attitude. He never got catty or angry or anything. He was really calm and positive and sweet." So, I think it is safe to say we have uncovered at least one component of the much talked about Saint Laurent NDAs, and it may or may not be bad behavior on Slimane's part. That part is actually still up for debate.