Final answer:
The defense mechanism exhibited by a person who starts finding fault with others after a bad day is known as displacement, where stress is redirected towards safer targets.
Step-by-step explanation:
When a person begins to find fault with everyone after experiencing a series of stressful events such as losing an important account and getting a flat tire, the defense mechanism being used is likely displacement. Displacement involves transferring inappropriate urges or behaviors onto a more acceptable or less threatening target. In this scenario, the individual's stress and frustration are being redirected from the actual sources of distress to other people, who are not the cause of the problems being experienced.