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.

Could even run the instance from your phone or whatever device used to look at Lemmy