It could be a variety of things, perhaps its to show that the character of Towers, while seemingly trying to get past the loss of his wife and children by being romantic with Moira, will not abandon the past, the crowd is like symbol of disapproval of their actions and interrupts there romance. That Towers will not abandon the past, is confirmed by the end of the film, when he chooses to continue his duty in the navy and sets off in the submarine. It can also be that while Towers and Moira seem to have found one last piece of humanity in loving each other, the others are a reminder that no matter what they do, humanity is on the brink of annihilation. Everything the characters do to try and be human, to try and live one last day, is interrupted by the reminder that the atomic cloud is coming and humanity's end is near.
Hope that helps some. I published quite and extensive essay on the film which you can read here: