The only stupid question is the one that goes unasked, as they say...
I got only two ideas.
- The name comes from the force of the dance, and a properly done Sundrop dance is like dropping the sun on someone. That is, it has great weight and power, which makes sense considering it's intended to be a war / deterrent dance.
- The name comes from a long metaphor. The sun, light, is hope, and by using the Sundrop dance you are destroying their hope. That is, the sun drops below the horizon, leaving only darkness and despair.
Might I add that, while it's highly entertaining to see the Vanu do it, the Sundrop dance is... significantly less powerful when my PC does it? Perhaps because I'm playing a waifish Auri woman, but...