[GAMEPLAY] Scuttler doesn't like being stunned

Was playing Sejuani, and i was taking Scuttler. I got it to four stacks and used my e on it, it started the animation and used the mana, but Scuttler didn't get stunned, like it lost all of its stacks instantly. I was able to apply the stacks to it again and stun it. I have never seen this occur before, and have no idea how to reproduce it. http://plays.tv/video/59dc18657f6810f0a6/riot-