John Horn of the Los Angeles Times has an extensive piece, in part about how “Dr. Seuss’ Horton Hears a Who!” director Jimmy Hayward used to be at Pixar, and how other people have left Pixar to pursue other opportunities.
This is not unique to Pixar, of course. Any time an organization, especially a business, builds a great reputation and develops great talent, that talent is going to be in demand, and they may have to go elsewhere to do some of the things they want to do.