To get mildly technobabble'ish, as has been suggested before numerous times, it's possible that the Hobus star had unique subspace properties (or such) that made it going supernova a greater threat than a normal star undergoing the same process. Then again, the supernova may have been caused artificially, which could also explain the increased danger. That might also explain why Spock's calculations were off.
In fact, prior Star Trek
movies have already given us precedent for cosmic explosions having effects that propagated FTL. First was the Praxis explosion affecting the Excelsior
in The Undiscovered Country
; I profoundly doubt the ship was passing through the Qo'noS system itself at the time. Second were the trilithium-induced supernovae in Generations
, which were shown to have instantaneous gravitational effects on the courses of vessels and phenomena parsecs away. So for better or worse, this is an established reality of Trek-universe physics.
As for Hobus being 500 ly from Romulus, I'd prefer to ignore that. Countdown
is not canonical, and frankly it makes scientific errors far worse than anything in the movie, like claiming that Hobus is one of the oldest stars in the galaxy. It's the huge, short-lived stars that go supernova. The oldest stars are tiny, cool red dwarfs that aren't capable of it. Not to mention what Countdown
claims about the radiation front somehow accelerating, which isn't in the movie; there, Spock just wasn't ready in time to save Romulus. Even the name Hobus was never mentioned onscreen.