It would be common to see the mother's maiden name used as a middle name (I've seen several families where a number of children bear the same middle name, and that's generally the mother's maiden)
Often the child is named after a relative, so Mary Smith Brown is named after her grandmother, Mary nee Smith. There are several suggested 'patterns' for first names - you might want to check if any of your families tended to match up with these:
http://www.users.bigpond.net.au/hardiehistory/namemeanings/namingsystems.htm#StandardEnglishAnother possibility is that the middle name is taken from the clergyman who baptised the child - this apparently was common in some areas.
Sometimes, of course, there's just no pattern. One of my grandfathers has the middle names 'John Rice'. His father was John Rice Tong, and the grandfather of John Rice Tong was also John Rice Tong. The mother of that John Rice Tong, Martha Pascoe, had a brother, 'John Rice Pascoe'. The paternal grandmother of Martha Pascoe and John Rice Pascoe was Jennifer Rice, who married Anthony Pascoe. However, none of the children born to Anthony Pascoe and Jennifer Rice carried Rice as a middle name.
Anthony Pascoe = Jennifer Rice
c.
Anthony Pascoe = Martha Heathrington (note this surname was not carried through)
c.
Martha Pascoe = Benjamin Tong and John Rice Pascoe
c.
John Rice Tong (no children with middle name Pascoe)
c.
William Henry Tong
c.
John Rice Tong
c.
Edgar John Rice Tong
In general, this particular family didn't have surname middle names, so I was happy when I tracked it down!