User blog comment:KaiserLos24/Cho'Gath Visual Update?/@comment-4846081-20140509233425/@comment-620927-20140510183530

The not getting knocked up from walking into the circle after it's started is intended - simultaneous knock ups from one ability all occur within one second, which means that somebody can't have a staggered knock up from the rest of the group's knockups.

It's a bit silly, but it makes sense if you land enough Cho'Gath Qs and just watch the enemies get flung up. However, I think it'd be totally fair if people that walked into the AoE after the knock-up still got slowed. Perhaps they could look at his Q and separate the slow component from the knock up and make it a part of the AoE.