Gameplay - Leona kills Tryndamere during Undying Rage

I was playing Leona top with Sheen, Glacial Shroud and Doran's Blade. I got Tryndamere low with autoattacks. He used Spinning Slash to escape, I used Zenith Blade on him, heard the yell of Undying Rage start, and watched him die instantly, before I could dash to him.

I have been unable to reproduce this in other games.