Why it happens
Fish has very little connective tissue compared with meat, and its muscle is made of short flakes held together by thin layers of collagen. As it cooks, that collagen turns soft very quickly, so the flakes come apart easily. Turning it too early, before a crust has formed, makes it stick and tear. Overcooking, a pan that isn't hot enough, and delicate fish such as cod or haddock all make it more likely to fall apart.