Why after a hard hit to the head some people speak with an English accent?

This phenomenon, known as Foreign Accent Syndrome (FAS), occurs when a person's speech is altered due to brain injury. It is not that they suddenly acquire a specific accent, but rather their speech patterns change in a way that may sound like a foreign accent to others. The exact mechanisms behind FAS are not completely understood, but it is believed to be a result of damage to the brain areas responsible for controlling speech.