When Skarner E collides, it checks a small area in front/behind the target to see if they're extremely close to terrain, so Kayn would get instantly stunned.
Kayn E is scripted to teleport him out if he ever receives a hard CC while inside a wall (IIRC).
Skarner exits the wall less quickly. His E buff drops, which makes him have to get out, which he does via a fairly quick pseudo-dash (IIRC).
Or they kiss.