Plummer talks about it in one of the extras on the Special Edition of the TUC DVD (although he left out some key details; he only said he was in the hospital).
The kicker was when people started visiting him in the hospital and told him just how great Shatner was (mainly how he was playing the part completely differently than how Plummer had been doing it), giving Plummer added incentive to get healthy and reclaim his rightful place.