In chapter 19, Tom Robinson takes the witness stand, and Atticus asks him to explain what happened on the evening of November 21. Tom tells Atticus that Mayella called him inside her home to help her put a door back on its hinges. When Tom enters the home, he discovers that the door is fine and wonders where Mayella's siblings are. Mayella then tells Tom that they went to go get ice cream. As Tom goes to leave, Mayella asks him to get a box down from the top of a chiffarobe. When Tom stands on a chair to retrieve the box, Mayella begins to hug his legs. Startled, Tom hops down from the chair, and Mayella starts hugging Tom and kissing the side of his face. Tom then shoves Mayella out of his way, hears Mayella's father threaten to kill her, and runs as fast as he can out of the house.
When Atticus asks Tom why he ran, Tom responds, "I was scared, suh" (Lee, 199). Atticus then asks Tom why he was scared, and Tom says, "Mr. Finch, if you was a nigger like me, you’d be scared, too." (199)
Essentially, Tom knew he was in a difficult situation the second Mayella made advances towards him. Tom knew that the racist community members of Maycomb would blame him for Mayella's actions and ran because he was scared for his life.