You're probably right that the rise in militia type groups is correlated with the rise in AR type weapons. I'm not going to look it up, I'll take your word for it, as a general correlation seems reasonable. However, we get into the old adage that correlation does not always imply causation. There are plenty of other factors that overshadow the rise of militia groups. They may not be intuitive for someone who is outside of the "gun culture," but I'll list a few off the top of my head.
1. Obviously repealing something previously banned will lead to an uptick of its legal usage. The rise in AR type weapons began as soon as the AWB sunset in 2004. It probably would have been quicker had the industrial capacity been there to produce them quicker. People naturally want what they can't have, and there was a real fear at the time that it would be renewed by the Bush administration if congress put it across his desk.
2. The wars in Iraq and Afghanistan, combined with a pretty high level of support and adulation of the military in the wake of 9/11. Our vets were heroes, so it was only natural to want to be like them.
3. The technological development of optics and the industrial base to manufacture them. We suddenly had an industrial base manufacturing large numbers of reflex sight like red dots and holographic sight that allow you to get on target quicker than ever because you don't have to carefully position your head on the stock to align iron sights or to see a scope. As long as you can see the dot on the target through the window on the sight, your bullet will hit there. It's like having a laser that points to your target, only visible at all ranges and vitually all lighting conditions. I've heard anecdotes that the number of head shots on enemy combatants increased dramatically as these types of sights proliferated in out military. This also happened to be useful for hunting in cases where a quick and accurate shot is needed. If you are hunting deer in thick brush where you may only have a couple seconds to get on target and shoot your quarry, these types of sights dramatically help your success rate. This also kind of leads into my next point.
4. Firearms manufacturers began to notice how practical these guns would be with modern optics, but the caliber of the typical AR was not suited for hunting most game other than varmints. But one of the strengths of the AR platform is that you can easily change the bullet it shoots with a new drop in upper as long as the new round isn't too long or too fat to fit in the magazine. They began to produce more and more rifles for rounds more suitable for hunting and even developed new rounds specifically to maximize hunting performance of the platform. Now the AR in suitable calibers is one of if not the fastest growing platform for hunting. To say that it isn't particularly useful for hunting is pure poppycock and can only be claimed by someone who isn't familiar with hunting with one in an appropriate caliber and with modern optics or someone who just prefers to do things the old fashioned way. I hunt with both an AR and a flintlock, so I do get the old fashioned stuff and go more old fashioned than most of the haters do themselves.