I agree with the fire theory. I know one awesome piggy-wiggy spot which is absolutely infested with them (grass ticks, a.k.a. kangaroo ticks, a.k.a. seed ticks). It hasn't been burnt for several years. It is the only spot like it that I've encountered in quite a few miles in the bush. Of course, we find the odd one elsewhere, but in this horror spot, a fellow can look down at his trousers, and recoil in abject terror as he spies literally two-hundred-plus of the little buggers. The next five days and nights are hell!