From: "This War is for Us," By: Pasko, Ariel Natan, IsraelNews.com. September 06, 2008
"Jewish festival celebrated in February or March (the 14th of Adar in the Jewish calendar), commemorating Esther, who saved the Jews from extermination by the Persian king's vizier (executive officer) in 473 BC during the Persian occupation. The festival includes a complete reading of the Book of Esther (the Megillah) in the synagogue, during which the listeners respond with stamping, whistling, and hissing to the names of the evil characters."
Purim Fest, 1946 – A Tale of Kiruv
March 23, 2008
... Streicher and the Nazis were close students of Jewish culture. They timed deportations and mass killings to coincide with Jewish holidays. They built anti-Jewish propaganda based on Jewish customs and observances – and one of their favorite holidays for this purpose was Purim.
Here's how Menachem Mendel tells the story:
In its August 1935 issue (no. 35), Streicher’s paper took up a story previously published by the Reutlinger Tageblatt about a Jewish chemist, Dr. R.F., who had been accused of torturing a cat to death. According to Der Stürmer, in order to kill the cat, F. had tied it up in a sack, which he then threw onto the concrete in front of his door.
Purim was a holiday which was important to the Nazis because it showed how bloodthirsty Jews really are.
The numerous confessions made by the Jews show that the execution of ritual murders is a law to the Talmud Jew. The former chief rabbi, and later monk, Teofite, declared that the ritual murders take place especially on the Jewish Purim in memory of the Persian murders, and Passover in memory of the murder of Christ. (Der Stürmer, no. 14)
In the Book of Esther, we read that in one bloody night the Jews slaughtered and destroyed 75,000 Persians. Even today, the Jew celebrates Purim to commemorate his great triumph. (Robert Ley, Pesthauch der Welt)
Purim was featured in the Nazi propoganda film The Eternal Jew and the Nazis saw Purim as part of a pattern of Jewish responsibility for tragic events in human history.
In other words, the Nazis were intimately familiar with Purim and viewed the holiday as an archetype for the Jews' supposed "bloodthirstiness."
Streicher, the publisher of Der Sturmer, by profession needed to be an expert on Jews and Judaism, and he needed to use JUdaism against the Jews. And so he did. Yelling "Purim Fest!" is not only unsurprising, but it fits perfectly with a lifetime of the man's work.