This isn't how evolution works. It doesn't work on groups, it works on individuals. So, unless high-T folks got less food than they needed, and thus did not reproduce as often, high-T-ism would not decrease in the population.
If it is true that high testosterone increases one's ability to compete for food and mates (which it does) then it should increase over all, since the high-T guys would reproduce more often.
So, having testosterone levels drop suddenly is a puzzle. There was probably some unrelated mutation that conferred a benefit, like disease resistance, that swept through the population, and that had low-T as a side effect. That would have allowed the small groups to coalesce into larger groups, due to lowered levels of competition.