Kates and her business partner Jennifer Allen were formerly publicists at PMK-HBH and left to form their own company when PMK merged with BNC in 2010. They have decades of experience and they represent some incredible talent. Among others, Kates represents Shia, Josh Hutcherson, and Chris Pine while Allen represents Christian Bale, Matt Damon, and Jesse Eisenberg.
There are a lot of reasons I could talk about for why I think this happened. But in sum, imo Zac wasn't making any gains in a media and image perspective, so maybe they were ready for a change. Whatever the reason, I'm definitely looking forward to what comes of this.