‘ And he took the people, and divided them into three companies, and laid wait in the field, and he looked, and, behold, the people were come forth out of the city, and he rose up against them, and smote them.'
Gathering that now that Gaal and his brothers had been expelled, the population of Shechem would feel able to move more freely, Abimelech divided his forces into three companies and waited in the countryside outside the city, and when many of the people in the city came out to work in their fields he arose with his men and smote them.
Alternately if we assume that Judges 9:42 speaks of some people who had already left the city, this tells us that more now left the city, and it was they who were first attacked.