Only Tom knew his reasons, and they died with him. Author Harper Lee does not reveal Tom's motives for running, but the fear of the unknown was one consideration. During his questioning during the trial, Atticus asked Tom why he had run in the first place when confronted by Bob Ewell.
"I was scared, suh."
"Why were you scared?"
"Mr. Finch, if you was a nigger like me, you'd be scared, too."
The walls and fences of the prison may have been closing in upon Tom, or it may have been the absence of his family; Tom was 70 miles away from Maycomb, incarcerated at Enfield Prison Farm in Chester County, where he was allowed no visitors. Atticus had always known that Tom would have a good chance to be released upon appeal, but Tom must have figured differently. It may have been a spur of the moment decision for Tom, who decided to make a
"... blind raving charge at the fence and started climbing over. Right in front of them."
But, as Atticus told Aunt Alexandra at the Missionary Circle tea just after Tom's death,
"I guess Tom was tired of white men's chances and preferred to take his own."