I have this in my family tree, too. Sometimes it can provide a clue to the mother's maiden name. My g-g-grandfather's middle name was his mother's maiden name. The same is true for his father's middle name.
My great-grandmother gave one of her sons an unusual middle name. I recently learned that it was her aunt's married surname. That couple got married on my great-grandmother's birthday. My g-g-grandmother died when my g-grandmother was about 12 years old, and I wonder if her aunt helped take care of her. In any case, I'm guessing that she loved this aunt & uncle a lot.
One of my daughter's middle names is my maiden name (which I kept when I got married).
Regards,
Josephine