Tahm - Skarner mark bug?

I recently played a game as skarner support against a tahm and I noticed that is he ate his adc after they had been marked by skarner's E the mark transferred to him allowing me to stun him with an auto. Then when kog came out and still had the mark so was able to pull off the stun once more and got a double kill.

Is Tahm designed to work this way or is it a bug?