The funniest theory of mine is that Nomads actually acted in... self-defence. Why?
Well, when I rewatched Extended Intro for a longer time, I've noticed that few fighters hit Suncrusher by accident, right after humongous Nomad vessel decloacked. Taking it as act of hostility, Suncrusher turned onto sun, fired its stuff and cloaked away, leaving Sol to its new fate.
Edit:
My other theory was that it was Coalition which gained access to some kind of probe and reverse-engineered Daam K'vosh technology. Sources for this theory? There's a mission in Starlancer, during which Alliance pilots witness a completely new structure, which is a jump-gate (interplanetar one, instead of actual long-range jumpgate) built and used by Coalition.
It could be the reason on why Nomads went on rampage to Sol.
