I think that it's hard to differentiate between "movements" and "groups." f64 had a battle to fight, so they wrote a manifesto. They were also the underdogs, the revolutionaries. But they were a group. When Magnum formed, it was largely an economic venture, but there were photographic rules. I understand that Cartier-Bresson was adamant about acceptable focal lengths. Both had great impact on photography, and I think both could be considered movements in hindsight. Plus, Magnum fought the economic battle for all photographers, and now we own our photos. There are a lot of groups like Magnum now, might they also be movements? They usually gather around shared principles or styles. Even publisher groups sometimes constitute a movement, like LIGHT & LAND, or CRAFT & VISION. And analog is itself a movement! I think we're pretty well organized and connected - but I wouldn't want a manifesto. Or it could be a one-word manifesto - film. And - prepare to throw things at me now - the lomography crowd actually has a manifesto! Love 'em or hate 'em, they stand for something.
Also, Ansel seemed like a "manifesto" kind of guy, a product of his time. Everyone was writing manifestos about everything back then.
I'm sure that my dip into this pool is pretty shallow, but I think there are movements, but some time is going to pass before we recognize them as such.