The answer should be a simple one: emotion. However, emotions can be manipulated by rhetoric, alcohol, the mob mentality, cultural mores and norms.
If a normally rational person is provoked beyond all reason, then that "sane" person will eventually become irrational in a fit of anger and passion defend his or her space.
In the story, we see Mayella Ewell caged like a slave to her father's whims. Yet, she finds a way to escape the savage conditions under which she is kept even though it cost Mr. Tom Robinson his life. Mayella acted irrationally due to conditions beyond her control.
Humans resist and fear change. The cultural dynamic of the time was changing. Atticus Finch upheld the law regarding the defense of Tom Robinson even though he realized that Tom's status as a colored man would limit the outcome of the trial to go against his favor. Atticus maintained his reason in the face of unreasonable circumstances and odds against him.
The mob who came to lynch Tom Robinson was probably the most telling example of how emotions can be manipulated by rhetoric and alcohol. The men in the mob all lost their reason due to the public outrage of Mayella's story and the addition of alcohol to fuel their rage. In a moment of clarity, Scout Finch struck up a conversation with one person in the mob whom she recognized as a normally rational, reasonable person. Her courage and bravery turned the tide and disarmed the mob mentality.
Bob Ewell is the one example of a person whose life is totally out of control and whose only ability to be in control is by controlling his daughter, Mayella. He is well known to the community as "white trash" and a trouble maker. His only standing in the community is that he is "not black." Having the nerve to blame Tom Robinson for the attempted rape of his daughter instead of admitting to the community that his neglect and drunkenness caused her to reach out to anyone in the community, even a black man, shows the reader just how unreasonable and irrational Bob Ewell has become. His lie is found out when he is asked to write his name; he is left handed.