Your favorite designer rips off other designers.
There's a very thin line between inspiration and plagiarism. When one designer does something that resonates with consumers, others are without a doubt going to follow suit to get in on what's caking. The resulting group of similar clothes may may be talked about as a trend but it's really just brands crowding in on the momentary fad by copying colorways, silhouettes, or entire garments.
Why do you think brands are hesitant to allow photography of just produced collections, or keep designs and practices close to the chest? If you think your favorite designer is only drawing inspiration from 1920's Scottish dockworkers or the retro-futurism of modernist Japan, please stop being naive.
Four Pins' uncovering of Pyrex straight jacking Rugby flannels (they think), Stubbs & Wootton firing shots at Del Toro, and the YSL vs. Louboutin throwdown are just some recent examples of call-outs going public, but this shit happens all the damn time. It may not be the Wild West of the black market, but intellectual property is a tough thing to defend when your products are out there for the world to see. All designers and brands can do is protect they neck as best they can.