Astronaut Sion Confirmed?

I was playing full attackspeed/crit Sion in a custom game (he does a little conga line dance as he kicks your enemies to death, and that's really what i needed to turn my day around). Anyway, I noticed that occasionally when I knocked up a champion or minion, they would just...hang there. They wouldn't drop, move, use abilities, or autoattack. The longest time was about ten seconds, the shortest was about three. I couldn't find a set of conditions that caused this consistently, however.