In my experience, the higher energy breeds are rehomed most often. German Shepherds, Border Collies, Pitbulls, Huskies, some Terriers and Poodles. Often the people that rehome them maybe have just looked at them online and thought they were cool or their family had one when they were kids so they decide to get one, but din't have any idea of what the dog's true requirements are. I mostly see them rehomed due to behavioral issues. Things like problem barking, chewing, hyperactivity, aggression. They're often not well socialized. I think the problem comes about because people often get dogs and only look at the breed through rose-colored lenses. They only want to see the best and want to disregard any common disadvantages or problems that come with the breed. So they never do the work to avoid those problems, don't want to work on them when the problems arise, and often assume they just got a bad dog. It's depressing how little thought some people put into the dogs they pick up on a whim, but I see it happen far too often every single day.