Sometimes it's impossible to find out why a surname changed. Surnames changed in many ways for many reasons, it could be that the family wanted to distance themselves from someone or something. It could also be that they moved to a different area and that their accent lead to registrars etc writing the name as they heard it and perhaps if the family couldn't read or write, they wouldn't know any better!
Sometimes, like in my family it can be due to illegitimacy, a child born to parents who marry later will likely be registered with the mother's name but will be known by the father's name.
Sorry to not be able to help much, but our ancestors certainly didn't make it easy for us!!