This is the BBC's take, which seems as good or better then anything else I have read in the past few hours.
"The Communist Party's mass meetings are normally highly scripted events, leading to speculation that the timing of Hu Jintao's departure might not have been an accident.
He attended the earlier closed-door session on the last day of the Congress, then cameras were allowed in for the final portion of the day. It was just after the cameras had set up that officials approached Mr Hu and indicated that he should go.
That said, the party doesn't normally air its dirty laundry in public. If this was a deliberate show, it would mark a departure from usual behaviour."
And this, same source.
"Chinese state news site Xinhua reported that Mr Hu had not been feeling well during the session. It said his staff then accompanied him from the session to rest, adding that he is "much better" now."