Star Trek Online: The Needs of the Many
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.
: A scientist explains the Hobus supernova wake travelled through subspace, damaging some systems within a 500ly sphere, destroying Romulus and Remus
To me, this sort of ST technobabble sounds reasonable.
STO mission Ground Zero
: Praetor Taris, under guidance of the Iconians, and her Reman allies, used decalithium to initiate the Hobus supernova. They destroyed Romulus on purpose. (They didn't care about Remus because the Remans didn't live there anymore.)