Your followers are probably impressed with your posts. So out of respect, they "follow" you.
Others do it just to gain your follow. So you may get a private message from a stranger saying something like: "Hey, member mark #13 for you, please do the same for me"
it's like spam. they want you to follow them in return and then they feel like with more followers, their posts and opinions somehow have more sway. very silly tactic, but it is how it works on i-hub FNMA.