When I update stories is completely random. Lately, I've been unable to write, so it'll be a while until I update. Look at the bright side though, I'm constantly thinking about my stories so that means I'm thinking up scenes and stuff to improve them. The more time I have to think, the better the stories will be.
If I unfollow your story it's either:
A. I've lost interest in that fiction as a whole.
B. I've lost interest in your story. (Most likely I have forgotten what it's about and I don't have the time to reread it. I have over 1k chapters on another site unread because their notification system either broke or changed.)
C. A combination of A. and B.
Most of the time, it's going to be option B. with the example listed.