It is said that this was changed to form a character with the connotation "the fire leaves" due to the fact that Honno-ji Temple has repeatedly been devastated by fire ('hi' in Japanese).
これは本能寺が度重なって焼き討ちに遭っているため、「『ヒ』（火）が『去』る」という意味で字形を変えているといわれている。