The Accident/What happens to Rienhold Zucker? What is ironic about this?

Rienhold Zucker wants Hans's seat at the back of the bus. Hans, too exhausted to argue, gives up his seat without argument. The switch is made and the bus loses a wheel. The truck roles and, ironically, Rienhold Zucker is the only casualty. Zucker inadvertently gave his life for Hans.