DNA Matches - ancestry

I don't have a parent to test.

When I had my DNA done I already knew who my father was but some do not, had I not I would now or at least know what family he came from. My high matches are all on my mother's side though.

I don't know how widespread this is but I am beginning to see on a group I belong to more and more people getting quite high DNA matches. They have started from the position they know all their direct ancestors but then find out that what they thought, because it was on paper is true, actually isn't leading to another direct ancestor entirely for part of the family.

I have DNA matches, not too high but high enough, that I have no idea where they fit in. There is now a massive group of them in the USA that all share ancestors and me and I have no idea where they fit in, this group gets bigger every day. I believe it was probably a descendent of a direct ancestors' sibling that fathered a child with someone that wasn't his wife but probably someone else's can't think of any other reason.