Why?
T. Ray hits Lily because he is furious with her for leaving the house. She has run away to find her mother's sister, August, and he sees this as a betrayal. T. Ray is a volatile and abusive character, and he lashes out physically when he feels threatened or out of control.
The head injury is a pivotal moment in the novel. It not only symbolizes the physical abuse Lily endures, but it also highlights the emotional trauma she carries from her father's cruelty. It is this injury, and the subsequent fear it instills in Lily, that pushes her to finally run away from T. Ray and seek refuge with the Boatwright sisters.