Factual error: As Ray is fighting the big inmate over the cornbread, Ray falls over, revealing a yellow patch on the bottom of his boot. That patch is the trademark of a Vibram sole. Vibram started using the yellow patch in 1965. The scene takes place in the 1930s.
Factual error: In the scene where the jail warden's grand daughter has her baby and he is shocked to see that he is African-American, he goes to the jail to find out who the father is. She had just had her baby but when he holds the baby up to each inmate, that baby is at least 6-9 months old.
Factual error: When Claude is standing outside the car while waiting on the current warden to pick up the new warden, he's looking at the different styles that have come to pass while he was in jail. It it supposedly the 70s but when Claude looks at the guy with the stereo he hears Maxwell's "Fortunate" on the stereo. This song came out in 1999, not in the 70s.
Factual error: When Claude and Ray are in the car driving to get the booze, you can see they pass a home on each side with a more modern white truck at each home. These truck appear to be from the 70's or the early 80's when that scene is supposed to be in 1932. (00:12:40)
Answer: Because he is gay. In the time this movie is set, being homosexual was not only an extreme moral and religious taboo, but it was illegal in most parts of the US. Biscuit is too ashamed of letting his mother find out about his relationship with Jangle Leg, so he runs, knowing the guards will shoot him dead.
roboc