I believe the system automatically sets you to follow the most active posters in order to give you a feel about how things should be going. You are then free to unfollow them and choose your own people to keep an eye on.
From what I understand, though it's not a practice I'm used to, when the site wasn't very big, people would just check their follow feed (the house icon on the top right) and read what everyone around was posting on there first. Now it would probably be more difficult to operate that way, I'd think.
Ok. It does seem like a nice system, but I would have preferred being told that they did it. I want to choose who or what I follow on my own. But when I u followed all of the ones I got defaulted to, there was a message on the "following" screen that made it look like the default was not following anyone.