Quite frankly, the more members a group has, the better it performs, and the more important it is thought of as being.
Admins, and I know from personal experience, like members who don't join many groups, because they typically join more matches. This probably makes you especially susceptible to recruiting.
It is troubling, though, that when you ask why, no one ever answers. I personally have an open door policy, and reply to every message from a concerned member. Some people don't really care though, they just want more people in their group, because that gives them power, and so if you don't join when you get the invitation, they don't care about you. It is very unfortunate, but the truth.
I frequently get invited to join groups by people I have never been in contact with. I do not understand why. When I ask, I do not get a reply. Why would strangers want me to join their group?