I can't complain because I've got over 7.5k subscribers over there and now get a multitude of comments with each update, but I will point out that losing subs is very normal for me. I think I've lost about 150ish over the last month, and each update means a sharp decrease.
I read that you can only subscribe to 100 comics? If that's true, then maybe people unsub so that they can subscribe to new titles.
My growth in sub count ebs and flows, I usually lose them, then I'll get promoted and I'll see a huge influx. Surely what's currently happening in the story has something to do with it too.