Well I can actually think of some comics here on Tapastic that I subscribed too, decided I didn't like where it was going, found it again later, realized that it wasn't going the direction I had assumed and actually looks really good, and I subscribed again XD
So I think in some of these instances in these comments it could be that; but that one person who subs and unsubs every week? I have no clue what his deal is.
I don't use the mobile app, so for the people saying that, I guess that could be it (seems like Tapastic should fix that, though, if it's so easy to unsubscribe without meaning too... makes me worried ;o;)
The only problem I've had similar to this was when I had 49 subs, someone unsubbed (which sucked 'cause I was watching my sub number like crazy then), and after I got 51, that person re-subbed. It could have been a fluke, but at the time I was like 'what a jerk' >o<