As previously mentioned, single-strand knots occur more commonly for people with curly hair since, in general, curly hair is more prone to tangling. "It's the curly texture itself," Sturdivant-Drew said. "[The hair] is already like a spiral, so if you don't get through the hair with a comb or brush, it will eventually start to mat/tangle up, and that's when the knots begin. When curly hair starts to grow out, the hair wraps itself around the follicle, creating a knot."
Additionally, fairy knots can also form when the hair sheds and gets stuck or wrapped around other strands while falling out.