Its a traditional thing not a new thing.
If you look through family history its very common as it was a way of keeping the family name of the mother's family or a grandparent's family name.
This meant that surnames became first names. Some grew in popularity and became first names within their own right and were used by some with no family history of the name.
Then there example's like Harrison, which would have originally a first name that became used more commonly as a surname. Harrison is Harry's son. Remember for a long time that surnames were not used at all. They were started out of necessity after the Norman conquest and the start of tax records.
If you had eight Johns living in a village before surnames were common place, how did you distinguish between them? Nicknames became more or less the only names people had. Names like Miller and Mason, were references to trade.
Again this is another point - its a modern invention to have a legal name. In the past people may have had more than one name during the course of their lifetime. They had no need to write a name down - and most were illiterate.
Its became more commonly used for middle names for a long time, but not exclusively. Middle names came much later than surnames again to distinguish between people with the same name. If you look through census and birth records you'll notice that middle names start to become popular around the mid 1800 after the industrial revolution and the explosion of the population. This is not a coincidence, it was out of necessity, especially since families called their children after their relatives meaning that it was very easy to have a considerable number of John Smiths in the same family.
In my family and in DH family the use of a surname as a middle name runs through several branches of the family, as recently as one of my grandmothers and one of DH's grandfathers. We decided to follow it for DS, who has four names (god you must hate that!), which is another revived tradition from DH's family.
The current trend is actually a revival of old traditions rather than something which is in anyway new. The idea that they are Americanisms is incorrect. Americans kept old traditions longer than many in England did (many were of Scottish and Irish decent it must be notice) and it didn't ever fall out of fashion in the same way.
Names tend to go in cycles of about hundred years anyway, with a modern twist on the old and some finding more favour than others. Why is Arthur popular and not Albert or Alfred for example. Instead they have more popular modern variations of Albie or Alfie.
People are not generally as original as they think. Its surprising how many people think they are picking something really unusual only to find three in their child's class and are surprised to find this out. (Thanks Mum!).
I'm a nerd