Cho'gath Knock up while stunned (Ulting?)

I just played a game where it come down to me and another guy. We 1v1'ed for at least 10 fights. That may have been the best moment I had in this game in a long time. Not one sided or anything. We went through 2 Roulette's and 2 Elder dragons (so maybe more then 10? I remember it saying stage 7-2 at one point). Side tracking aside, I had a Leona in the front line and I noted that every time she would ult, Cho'gath in the center, He would still use his ult. Weather his mana was at 0% or 90% when he got stunned. As soon as he hit 100% he would ult. Sever times would leona stun happen and then half a sec later he would ult. I'm not sure how long Leona stun is for but I figure it would be for at least 2 seconds. Anything shorter and whats the point?