Final answer:
The atmosphere of the city becomes tense and people become suspicious and nervous due to major incidents such as terrorist attacks or violent crimes.
Step-by-step explanation:
The incident that causes the atmosphere of the city to become tense and people to become overly suspicious and nervous is typically when there is a fearful calamity or a major event that threatens their safety or well-being. This could be a natural disaster, a terrorist attack, or a violent crime.
Examples of such incidents include the terrorist attacks of 9/11, the Boston Marathon bombing, and the fatal police shooting of Michael Brown in Ferguson, MO. These incidents create a sense of fear, uncertainty, and distrust among the population, leading to heightened levels of suspicion and nervousness.