Why is my bank charging me a maintenance fee?

Hello there, I'm curious about something. I've noticed that my bank is charging me a maintenance fee on my account, and I'm not quite sure why that is. I've been a long-time customer, and I've always managed to keep my account in good standing. Could you perhaps explain to me what factors might contribute to a bank charging a maintenance fee, and if there's anything I can do to avoid it in the future? It would be greatly appreciated.