I agree with the posts above - seems that Porte has gotten into this 'I cant win, maybe I should go home' idea.
Look at Alberto - battled on after his shoulder popped out twice (most would have called it quits after the first), wasn't about to go home unless he couldn't hold onto the bars - and look where he is now.
Porte should look at his compatriot, Cadel Evans, who broke a bone in his elbow in the TdF in 2010, lost a heap of time as a result (as well as the yellow) in the mountain stage that followed, but refused to quit and rode on to Paris out of respect for his team. Evans is not the only one of course who kept going when all chances were lost, just one example. Others though, seem to pack up and go home when its all too much for their ego.
Hopefully he thinks better of it and starts tomorrow, rides for his team, and doesn't pull the pin on the rest day.