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.
Or it could have been something that would have been tweaked if the Writers' strike hadn't occurred.
In any case, it doesn't seem especially more grievous than numerous other Trek...anomalies.