Shalom <private> Freeman (Taylor),
you will have to be more specific. What do you mean by "posted on our tree"?? You are connected to the so-called "Big-Tree", which has 55.7 million people in it (and counting... http://www.geni.com/worldfamilytree ).
Sharleen, you can't follow an entire tree, so it's likely that they're only following a profile (or maybe a few profiles) of interest to them for research purposes.
Speaking just for myself, I often follow profiles that are from the same town as some of my ancestors in order to see if they'll connect at some point, which they often do eventually. I also often follow profiles connected to Geni projects with which I'm involved -- for example, I follow many Puritan profiles since I'm involved with the New England Puritans project.
We really can't answer questions for those users about why they might be interested in follow specific profiles, so maybe it would be best for you to contact them directly and tell them which specific profiles you've noticed them following. I can't speak about Ms. Rossing, but I do collaborate with Kenneth Kwame Welsh, (C) and I know that he's a very nice man who I'm sure would be happy to answer any of your questions. If anything, I imagine that he might be happy to help you expand your tree or the profiles he's following, so he could be a great collaborator for you.
Finally, it's important to remember that someone following one of your public profiles does *not* mean that they have access to your private profiles (which are your most recent ancestors), your family group discussions, or any other private information. So you don't have to fear any followers. :)