Paul Funston, Southern Africa Regional Director for Panthera, a global wild cat conservation organisation, also agreed that this incident is surprising.
In the wild, groups of lionesses do attack lions, typically in defence of their cubs or territory, and such incidents have been filmed at safari parks. However Mr Funston said he has never seen an instance that has ended in a death.
Wild male lions will also typically chase off any male cubs when they grow up to ensure they are alone with the pride lionesses. Sometimes the lions will kill cubs - usually when they take over new territory from another pride - to stake their claim on the females.
Male lions have also been known to get aggressive with females and can kill lionesses who refuse to mate.
One possibility, Mr Funston said, is that Zuri - who is described by the zoo as "an attentive and protective mother" - became fearful of Nyack, which led to the fight.
Zuri's natural instincts could have taken over at that point, he explained, and so she ended up killing him.
"Even if animals are calm or seem to be calm, it doesn't mean that there aren't underlying tensions," Mr Funston said.
Bruce Patterson, a researcher at the Field Museum in Chicago, said he knows cases of wild lionesses that have attacked - and injured - male lions who upset them. "[But] unlike the zoo case, no one went for the throat!" he said.
Mr Funston, who has studied lions for 25 years, acknowledges this is "an unusual" and "rare" incident - but that does not mean it is is necessarily strange.
"We see a typical model and we tend to think we know it all. But this is a highly socially complex species."
"That's one thing I love about lions," Mr Funston added. "You don't quite know exactly what's going to happen in a particular scenario, and that makes them really interesting animals to observe and want to protect and conserve."