I think it might be load distribution. I've been running the same pack for 10 years. I usually do about a dozen trips a year and some trips I get some nasty bruises on my hips, and others you couldn't tell I was wearing a pack. Still haven't figured it out.
Getting a low profile pant belt is what saved me. I just went backcountry last weekend with a thick belt and was reminded with bruised/chaffed hips that the thin low pro belt is key for me.
The front of my hip bones is usually a problem for me. I'm about a 29"-30" waist and the small belt fits well (28"-33"), but it's not perfect for my preference. I ended up ordering a small-medium belt and although I'm almost bottomed out near the buckle, it's super comfortable. Probably not correct, but I can hunt all day without any issues.
I had the same issue and posted it here some months ago, Aron saw the post told me to call him.
Here is what he told me personally:
Some aren't used to the Kifaru waist belt support. It can take a getting used to.
I told him when I raised the waist belt up all the pain went away. He said some guys have to do this, just remember to adjust the shoulder straps lower to accommodate this higher than normal position.
After seeing the pic above in this thread with the guy and the Tahr, I realized I was simply trying to run the waist belt too low. I was only off an inch, maybe two, but it was enough to pinch the hip.
Now I run the belt just slightly higher than the naval rather than thru its middle, and I am good to go. The belt is still getting support from the hip and keeping the majority of the weight off the shoulders.
When I am a few miles in , the hips sometimes hurt, I simply need to recinch the belt (same as a horse saddle) and the pain goes away.
I ended up buying a 26" hunter and played with the adjustments. I also have been very conscious of my clothing and haven't had an issue since I made the original post. Hope that helps you out as well.