By Hashem choosing the month of Tisrhei to judge the world, it demonstrates His great compassion.  He judges the entire world at a time when He is favorably disposed and full of mercy.  For in Tishrei, Klal Yisrael is busy doing a great many mitzvos: shofar, Yom Kippur, sukkah, and lulav, all of which arouse Hashem’s abundant compassion.  ~ Abudraham