In October 1938, the Polish government announced that they would refuse to admit Jews without valid passports if they had lived in Germany for more than five years. Many Jews wanted to return to Poland because of the anti-Jewish measures in Germany. To renew their passports, they had to get a stamp at the Polish consulate, but in many cases, they were denied the stamp. As a result, they were unable to leave Germany.
This was inconvenient for the Nazis: they wanted as many Jews as possible to leave the country, but many of the Jews in Germany were Polish. And so, Reinhard Heydrich, head of the Gestapo, decided to banish all Polish Jews.