Why does airpod automatically read short messages and nail messages?

Because of the automatic broadcasting function.

You can open the phone settings after connecting Bluetooth. Click the notification to turn on the Siri information broadcast. After receiving the message, it will automatically broadcast the message content. If you don't want this function, just turn off the Siri information broadcast.

Siri voice broadcast function is a new function of iOS 15. Before iOS 15, voice notification was basically useless, and now the updated voice broadcast has become a bit useful. After connecting a headset that supports voice broadcasting, you can listen to the new notification content that Siri automatically broadcasts to you when the device connected with the headset is locked.