Of course there could be sexual selection at play! I would argue though, that form of selection will take at many generations to take hold where as genocide of a species happens in a single generation. Of course, the smaller the population, the faster sexual selection will take effect but it might be hard to disambiguate with other fitness factors.

Fwiw, a very similar thing happens during selection for bacterial resistance. You eliminate all the cells susceptible leaving a population of resistant cells. This population then grows to replace the previous generation. You could analogize this with the elephants. Small tusk and docile elephants have a "resistance" to being murdered by poachers. The poachers eliminate all the others and suddenly the population seems domesticated.