I hunted in NG 2 a while after the first mortalities were reported. One of our trackers was a very sharp Herero man with a unpronounceable name. He lives in the area and has a Moraka where he keeps own livestock. A person with deeper understanding and a bit of substance, a man whose opinion I would listen to.
He gave me the most plausible explanation I've heard up to date.
According to him overgrazing in the Delta cause cows elephants to drift into the desert were they eat plant species that are unkown to them. Johan Calits showed me the overgrazing in the Delta on a earlier visit, and bemoaned the loss of the edible tree species. So the shifting grazing habits seems plausible.
I asked the Herero to show me the plant that kills the elephants, which he did. Gifblaar (Dichapetalum cymosum), a highly poisonous prostate shrub.
The explanation and the location and nature of the mortalities seem to fit.