The human brain is a really weird thing:
In November 1999, 57-year-old Judi Roberts of Sarasota suffered a stroke that left the right side of her body paralyzed. She was also unable to speak. After months of physical therapy, she was no longer paralyzed and was able to speak with some difficulty. Her speech gradually improved during the next year until she was speaking with the same fluency as she had before the stroke. However, instead of the familiar New York accent she once had, she spoke with a British accent.
Turns out she had Foreign Accent Syndrome, a "disorder linked to stroke-related or other internal brain injuries that leaves affected people with a foreign-sounding accent." Fewer than 20 cases have been reported since 1919.