Thank you for posting your curiosity in the Community. I would be inquiring as well if I see such a sign in prompt. Do you see the sign in prompt on your device's browser? Do you also lose the Internet at the same time? What's the model of your Rocket Hub?
Most likely your Rocket Hub loses the network connection and you see the sign in prompt on the device you were using at that time. When you power-cycle the Rocket Hub, it establishes the network connection and your devices get connected to the Internet.
If you were to sign in then you should see the Rocket Hub's admin console where you can manage and configure your hub. You absolutely don't have to sign in and continue to power cycle the hub when you see the prompt.