If you search these family branches it is likely the middle names will occur as an ancestor's or a relative's surname in previous generations. I have seen middle names given to children based on more distant relatives, even the surnames of brothers-in-law in one case, or even friends of the family, but that is rarer.

Middle names being set to surnames seems to be a habit that I see more of in Scotland, but even there it seems to be confined to certain families (in the broadest sense). Typically, my lot would use a surname as a middle name that was a maiden name for a close relative. As more children appeared in a particular family, then they would run out of close relatives' names and so (maybe) the later children had no middle name.

My GG-GF had 6 children from his 1st marriage. All had middle names based on maiden names for relatives (in no particular order) but the last child repeated that of the first.

The 5 children from the 2nd marriage got a bit more difficult. The 1st two had middle names, then they must have given up because the next 2 only had one given name - only the last reverted to having a middle name but that was really because she was named for her aunt who already had the middle name.

So really, not a lot of reasoning behind the choices - just a case of "It seemed like a good idea at the time!" 😉