We tend to envision Lot's wife becoming a salt pillar as a (lehavdil) medusa-like sudden freezing from a normal human-organic state to a stone-cold hard mineral state.
Was this incident a miracle?
This clearly implies that the fact that she and the rest of Lot's family were not getting hit by this downpour was a miracle. By looking back at the suffering of S'dom she lost the the privilege of this miracle so Hashem removed His divine protection from her and therefore she was hit.
Lot's wife becoming a salt pillar was a natural event (given the miraculous disastrous circumstances) that was prevented until then by super-natural means.