Where I live in east central Miss we have allot of those. Definitely genetics. Yes, an injury can also cause an antler deformity, but when you have lots of bucks showing up this way, genetics is to blame.
We hunt on a restricted access area of about 8000 acres, document each deer killed very carefully. Of the 30 rack bucks killed last year only 8 or 10 had what could be considered "typical" racks. Probably another 8 or so that were considerably different from one side to the other and the remaining 10 or 12 were like the ones you are talking about, "crap racks".
My 7 year old killed one of the few typical racks with a nice 8. I killed 2 good size bucks, one was a 10 point, was an awesome rack on one side, thick as your wrist and great color. The other side looked like a fricken moose antler. Had it looked different-cool as opposed to different-crap, I would have had it mounted.
We are telling all our hunters to kill any buck w/a crap rack, we need to get them out of the gene pool. I was going to let the second buck walk when I saw him coming toward me, looked like a nice 2 1/2 year old symetrical basket 6, until he turned and looked my way. Yup, one side a nice basket 3 on the other, 3 points Satan would have been proud of. Darwin never thought about an encore when it came to survival of the fitest.
We know for sure of one buck that is probably 5 or 6 years old in the northern end of the property that is one of the worst deformed of them all and he is spreading the crap horn gene, he has a beautiful 5 on one side and a single horn going straight up for about two feet on the other. But, he has proven that just because his horns look like crap doesn't mean he is stupid.
Just remember, it isn't just the bucks that have the gene too, it is the does as well. Keep the turnover rate high.
R
Hank