It's part of the fish's genetics. Not just flowerhorns but the purebred fish that were used to create them to. It's the same reason as if I keep midas I want it to have a big head. Fish with big lumps on their heads are usually the best glass bangers in my experience. If I'm going to keep flowerhorns I want to keep the nicest/best quality ones. Watching a flowerhorn grow a giant nuchal hump is part of raising them. It has a lot to do with genetics but the better condition you keep them in the bigger their head is going to grow. It is rewarding watching them grow a huge head because you know you're doing something right.