Congratulations!!!! It's a very difficult hunt, but a very rewarding hunt.
In regards to your broken horn question, there may be several factors that are involved as to why you see some many broken horn Oryx. In my opinion selectivity is one of the major factors. For over 30 years hundreds and hundreds of hunters have harvested typical trophy animals and have passed on non-typical animals, which leave more broken horns to harvest especially in the last 4 to 5 years as the population has been reduced.
There also could be a mineral deficiency, genetic and or environmental factors as well. They also can break their horns by fighting. I've seen several Oryx with partial horns imbedded in their body. To the best of my knowledge fences are not the reason why there are so many broken horn Oryx. Does it happen, it's possible, but it's not the contributing factor in my opinion. The biggest environmental factor that can be measured was the deep freeze of 2011. For the first time I saw 1 to 2 year old Oryx with broken horns. The freeze in my opinion was a huge issue for Oryx. The recruitment that year was horrible. The prolonged freezing temperature basically froze the bone cores, killing it, making the horns brittle. The freeze also created many earless Oryx due to frost bite. If we have multiple periods below freezing or subzero conditions you can almost guarantee they will be affected. Although they have thrived, they weren't made to withstand sever winter conditions. I hope this helps.