Why did my red hair turn brown?

without dyeing it, when I turned 19 my hair went from a rich vibrant red to a very (ugly) brown shade.

What would cause this to happen

The genetics and ontogeny (changes that occur during an individual's lifetime) of hair color is not fully understood. Human skin color and hair color are the results of the presence of light-absorbing pigments, called melanin. There are two types of malanin: eumelan (brown or black) and phaeomelanin (red). Your hair is red because the pigment phaemelanin is deposited on the hair as it grows out of the follicle. If your hair is turning brown, it means your body (for whatever reason) is now making and depositing eumelanin on the hair on top of the phaeomelanin. People with dark brown and black hair also have eumelanin deposited on their hair as well. Therefore, your hair is darker simply because more pigment has been deposited on your hair. The reason why that may be the case for some people is still a mystery. It may have to do with different genes being turned on or off as you get older.

You are usually born to produce a lighter tone of your natural hair, your hair gets darker as you get older usually when you go through puberty.