What's caused you to drop a neighbor?

Lag. For whatever reason, some neighbor towns become very difficult to navigate because of a lag issue in their particular Springfield. If they've been a good, long-term neighbor I'll hide their town for a month or two and check back in on them. If the situation has resolved itself then it's all good. However, from what I've seen, most neighbor towns that start lagging rarely ever stop lagging. When that's the case, I have to drop them.