News reports have it that Aamir, who was supposed to arrive at the event, chose to cancel it at the last moment, which eventually put the organizers in a big fix. Not knowing what to do, they then approached Big B at the last moment. Big B graciously agreed to do the unveiling at such a short notice. The organizers of the event, who confirmed the same, said that Aamir was supposed to come but backed out at the last moment, stating that he had some last minute dubbing to do because of which he could not come.
On the other hand, a spokesperson from Aamir's office refuted the said claims by stating that Aamir did not confirm his presence at the event, and hence, there's no question of him skipping the event.