I'm no expert, but I believe they can be caused by many different things. Genetics probably play a huge role in the development of a drop-tine. Also, I have heard that injuries to a deer can cause antler abnormalities (drop-tines etc.), but I am not positive on this. If it is a genetic thing, it would seem that they would keep the drop tines throughout their lives. If it is due to an injury, it seems possible that it could be there one year and gone the next.