It's been known since the 20's that some of the stones came from Wales, (although the the view on which quarry has recently been revised [1]), so it shouldn't really come as a huge surprise that some of the people involved in the mind boggling task of transporting them should also have originated there.
I really think the reason why might be the same as why I've spent ten hours reorganising my emacs configuration this weekend. Why not?
What really makes me wonder is how much of the output of people like us will be recognised in the far future. How much will be lost? Probably a tiny fraction will be remembered. Then it makes you wonder about what has been lost if we're only seeing a tiny fraction of their civilisation.
As for why, or how -- that remains a mystery.
[1] http://www.ucl.ac.uk/news/news-articles/1215/071215-stonehen...