Say Mr. X social media adds a few of your publicly listed friends. And some add him back. Then Mr. X adds all their friends by looking at their circles (both ways). This includes unlisted people. They add all of them. Again, some of them add them back without double-checking. This is reinforced when Google+ shows contacts in common. You think, hey this is a friend of a friend. Often they have more than 2 contacts in common.
It's either spreading like fire or my friends are particularly vulnerable to this, wich I doubt as most are tech-savvy.
I hope Google takes care of this issue before the Zyngas creep up and make the service as trashy as Facebook.
Edit: reworking examples to make it a bit more clear.