question: why would a surname totally change in the middle of a long line?

[HeiressofBickworth]

8. For one thing, spelling means nothing

So if the name sounds the same, the spelling doesn't necessarily mean it's a different family. Standardized spelling didn't exist, and there are examples of names sounding the same but spelled differently. This often happened when a person came through an immigration point. The person writing down the name spelled the name in the manner they thought is was spelled but may have been incorrect. Same with census takers. For example, if someone of German heritage wrote down the name "Deal" it likely came out "Diehl". Someone from England may have written it "Deale". One of my friends who is researching the Deal name, came up with documents when taken together showed six different spellings of the name, but all referring to the same family.

Also, as one poster said, socioeconomic status played a role. I have one ancestor without a title who married a woman whose father had a title and no male heirs. He took his wife's name (and title). And ever afterwards, the family carried the wife's name.