A baroclinic cold core storm became weakly warm core then underwent strong intensification because of strong SST anomalies, in a region with low vertical wind shear. Rare medley of events that allowed for it to happen.
I lied. SST anomalies are not why it strengthened. It strengthened due to mostly atmospheric pre-conditions: a shortwave trough that enhanced an existing area of low pressure. Became a warm core storm as the fronts dissippated and went over only moderately warm water