Why had Hans not previously joined the NSDAP as the majority of his neighbors had done? why does he change his mind and reapply for membership now?

In 1937, Hans applies to join; afterwards he sees a Jew-owned store vandalized and grafittied. Over the owner's objection, Hans offers to repaint the door. Angry over what he has seen, Hans punches through the door and window of the Nazi Party office and tells a member that he cannot join.

http://www.gradesaver.com/the-book-thief/study-guide/section5/

After the departure of Max, Hans has lost his optimism. He no longer plays the accordion and eagerly awaits his punishment for helping the elderly Jew on the street. In early November, Hans' application to join the Nazi Party is approved, several years after submission. Two days later, Hans is drafted into the German army, which is desperate for new recruits following severe losses against the Soviet Union.

http://www.gradesaver.com/the-book-thief/study-guide/section9/