A Metal Deficiency
The departure of SN 1987A from expected Type II behavior
is thought to be related to it being deficient in metals relative to
young stars in the Milky Way Galaxy. (Generally, the Large Magellanic Cloud contains lower
metal abundance than our own galaxy). This alters the opacities, which have
little influence on the core and the supernova mechanism, but do influence
the evolution on the HR diagram.
The modified evolution
caused the progenitor to move back toward the
main sequence after helium ignition.
It was returning to the red supergiant region after igniting carbon
when it exploded as a blue supergiant.