In my tank it is usually due to acclimation failure. The display tank I have now has been up 6 years and it has rock and coral in it that are probably 20 years old. It's a mature stable system. The fish that are in it are mostly several years old and of course they have their pecking order. Any newcomer is low man on the totem pole. For that reason I acclimate new fish in my sump where they can adjust to the new water parameters without harassment from the current residents. I also try to add at least 2 and if possible 3 new fish at once to reduce bullying or at least spread it out. This has worked reasonably well but it's not sure proof.