Gameplay - Mordekaiser loses some movement speed when activating Q?

With my rune and mastery setup, I start the game with 349 movespeed on mordekaiser. I noticed that after activating Q and then hitting something I lose 9 movement speed, which last for maybe 5-10 seconds even if the Q was used up and on cooldown. Is there something I am missing and this is intended? Sure can't find anywhere stating that he loses MS for that ability.