I may be biased as a BC grad but I dont necessarily blame Kiwanuka (sp) for pulling up as much as I am blame the officials for calling roughing the passer on any little thing.
The reason I mention this is my guess is that Kiwa had his head down and somehow thought that VY had thrown the football - if he tackled him after he threw it - the official most likely would have thrown the flag for roughing.
Again, I am not sure on this but just my two cents from watching the play.