When Cho'Gath has his 'spores' skill, he shoots damage to units behind the primary unit he's attacking. If those spores happen to splash onto a champion, the towers will target him and shoot him.
This means that you can't possibly try to last hit creeps near a tower if there is an enemy champion nearby, for fear of being murdered by the tower.
© 2014 Riot Games, Inc. All rights reserved. Riot Games, League of Legends and PvP.net are trademarks, services marks, or registered trademarks of Riot Games, Inc.