What is unusual about the old man sitting on the pontoon bridge at the beginning of the story?

The old man's actually sitting by the side of the road in the dust. What's unusual about this is that he's not showing any sense of urgency. The fascists are about to arrive at any moment, and thousands of people are already making their way to safety across the hastily-constructed pontoon bridge. But not the old man. He's just sitting there, seemingly trapped in a state of inertia, unsure of where to go or what to do.
This initial impression is confirmed by his subsequent chat with the narrator. The old man can't go back to his home town of San Carlos, nor does he feel able to go on any further. He's been so deeply traumatized by the displacement of war, not least being forced to leave behind all the animals he once cared for, that he's pretty much given up the ghost. So he continues to sit there in the dust while thousands of people frantically make their way across the bridge.