Do not assume that an ancestor who has a surname as a middle name got it because that was his (or her) mother’s maiden name. Henry Johnson Smith might have gotten his middle name from a non-relative whose name was Johnson.
And George Washington Jones’ mother probably was not a Washington.
Surnames as middle names may be clues as to connections or they may be something else altogether.
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Also, check for naming patterns. My brother's middle name is Dad's first. Dad's middle name is hid dad's first whose middle name is HIS dad's first….
And, sometimes those were grandparents' surnames. It was very common in the Old South to give surnames as given names to both men and women. My great grandmother was Elmina Mayo Riddle, after her grandmother Elizabeth Mayo.
My grandfather's middle name was Hinson, which was the surname of a friend of the family.
Sometimes the middle name is the last name of a prominent citizen in the community or maybe the doctor who delivered the infant.
A really unusual first name could have a connection with a previous generation. While working on my husband's line, I found an ancestor with the first name of “Artipe.” I had no other real clue about this man, but I had about four possibilities using his surname and the time and place. When I got to the third line I was investigating, I found a grandmother named “Sarah Artipe.” Sure enough, this was the family. They had turned a surname into a given name.