“Rabbi Elazar said: Those on the path to perform a mitzva are not susceptible to harm; neither when they go nor when they return.” The reason they’re not even susceptible to harm on their return is because even after one does a mitzva the aura of the mitzvaenvelops them. R. Zaitchik connects this idea to the days that follow from Yom Kippur. During this period we return from Yom Kippur and we are less susceptible to the spiritual harm of the rest of the year. The midrash adds another reason. This is the busiest time of the year as we build the sukkah and procure the four species. When a person is this busy, no harm comes their way!