why does Hans change seats in the military vehicle? why is it very fortunate for him?

Hans changed seats in the military vehicle because he was left with little other choice;

"It was a surprisingly clear afternoon and the men were climbing into the truck. Hans Hubermann has just sat down in his appointed seat. Reinhold Zucker was standing above him.

“Move it,” he said.

“Bitte? Excuse me?”

Zucker was hunched beneath the vehicle’s ceiling. “I said move it, Arschloch.” The greasy jungle of his fringe fell in clumps onto his forehead. “I’m swapping seats with you.”

The seat change was fortunate because the vehicle blew a tire and careened out of control. The change in seat saved Hans' life.