Really, if your instance is just for you and your account (maybe a few friends, family), and you're not running any communities on it (all your posts and comments are on other instances), there's really no reason anyone would defed you. Unless you're a dick.

I have to think that if your very small private instance was federated with a large public instance, your instance would pull down content from the public one. That's bandwidth, and that's costing the large instance some amount of money/performance. Because your small private instance isn't "giving back" a level of content that the large instance would want or need, that could be the unpredictable reason you mentioned.

Now, I don't see that being a real issue unless there are so many low-content instances that Large Instance reviews their usage and finds out that an unacceptably high portion of their bandwidth is being used by "leeches," and wants to control cost.