Two traditions I know of regarding middle names (I don’t think they’ve been mentioned…)
-Jewish one where you give either a first name or middle name linked to a family member who has passed away
-French one (probably more universal than that) where your middle name is the first or middle name of your mother/father or grandmother/grandfather.
A third one I almost forgot: a lot of Catholics from a few generations back at least had the middle name “Mary.” My uncle, for example, is Georges Marie Bonnet. My mom has Marie as well, along with two different grandmother first names (Claire and Genvieve). I was given just the Claire bit as a middle.
Another possible reason for the rise of middle names: There is a tradition among some Catholic communities to give children the names of saints. I think there was a recent thread on it. Sometimes, you wanted to give them a name that wasn’t canonized, so you’d add a saint in the middle.
I was born to a Catholic family in Boston (I’ve since recovered with few ill effects), and they wanted to call me Eric. Since there is no Saint Eric, they gave me the middle name Joseph.
