Most deer with one messed up antler will grow normal antlers the next year.

It can happen due to injury or sickness during the growing period.

Lots of times bucks lose an eye in the rut the year before then have a messed up antler on that side the next year.

Genetic defect would be the last and least likely of any of these scenarios to be the cause of the things that you are seeing.

I've watched a deer for 5 years that is huge. Last year we were planning on taking him (he was 7.5) but when we found him he had a huge spike on one side. We let him walk. This year we have had a ton of rain, and he is bigger than he has ever been. You would have never known he had a huge spike on one side last year.