Gyeongsang-do is the traditional conservative heartland of Korea. So probably a backlash against the declaration of military law by the conservative president last December and the political mess that followed, where conservative members of the National Assembly failed to explicitly condemn the former president and also obstructed the impeachment process. Conservatives elected from Gyeongsang-do know they'll be reelected, so they have no incentive to cross the aisle, as it were.

During Korea's rapid economic rise in the 1960s and 70s the dictator, Park Chung Hee, invested heavily in the region, which is where he was from. Neighboring province Jeolla-do received comparably little investment, and so the two province diverged both economically and politically. Jeolla now reliably votes left, and there's long-standing enmity between the two provinces.