Hecarim kills Kayn in wall possible bug

When Hecarim uses his E his auto attack range increases from 175 attack range to 325 (according to the wiki), so it's entirely possible you were within the increased range. You were also moving away from during Hecarim's auto attack animation, but because the auto attack was already locked to hit you, it may have 'artifically' increased the range, like how sometimes a melee attack hits you despite being away from it (There are frames where the auto attack will hit you unless it's interrupted).