Final answer:
The people of Grover's Corner may have locked their doors due to external threats, a desire for privacy, or changing social dynamics.
Step-by-step explanation:
One possible explanation for why the people of Grover's Corner locked their doors, even though they all knew each other well, is the presence of external threats or dangers. The passage describes instances where the cable car stops and the street becomes still at night. This suggests that there may be instances of crime or danger in the neighborhood, prompting the residents to take precautions.
Another possible reason could be a sense of privacy or a desire to maintain personal boundaries. Even though the residents know each other well, they may still value their privacy and feel the need to protect it by locking their doors.
It is also worth considering the evolving nature of society. While the residents may have known each other well, there may still be a degree of mistrust or fear due to changing social dynamics, making them inclined to lock their doors for added security.