Fear.
A very good point. I think you are right, certainly among those who own private arsenals or powerful rifles. Not all, perhaps, but a good many of them.
As for what frightens them, well, there might be many things. Each other, perhaps. Some appear to be frightened some "unconstitutional" hard-line government might develop, so they need arm themselves to resist it; but that's probably only a minority of the gun owners.