[IanClarke]

The guys weren't cutting it any more. They were not delivering on the expectations of the group (growing it). Life became too harsh and stressful for them, they are not even mating any more, since the offspring couldn't be fed, so they stop breading with the females. Still: Everyone is disappointed and the males know it, too... affecting their behavior which gets less dominant/aggressive. The females are now in focus, since they procreated more reliably (on average, the old alpha, that had a lot of offspring, is likely dead by now). The shift to a female leader means the population is in trouble and in decline. A crisis manager(female) gets installed.

Note that typically older males mate with (much) younger females: It's no coincidence, that she is that much younger than her male counter parts: She is angry about her mating partners that they have stopped mating (with her). Older females have offspring. The old alpha male, which is dead, has a lot of offspring. The external life conditions just became too harsh, this is not a happy setup. There are 2 strategies, but one is to react on external circumstances (stop procreating), instead of blindly going ahead and having to stem the bill later (starving babies). Apes are intelligent enough for the first, but there are furious women now (including "her").