I understand the rhinoceros having difficulty nodding, but the soldier having trouble responding?

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A black rhinoceros nearly goes into convulsions trying to nod at a visiting
soldier.
A visiting soldier nearly goes into convulsions trying to nod at a black
rhinoceros.

The mob, in this case, is merely mimicking you. If you curtsied at that soldier, he would curtsey back, even though he is not wearing a skirt.

We've been gradually improving socials and how mobs react to them, but this is the sort of thing that we could only prevent by having mobs never echo a social back at you.