Why does my iPhone 13 screen go black after charging all night?

iPhone on charger all night. when I wake, screen is black, I press, volume up or down and on/off button, emergency alarm sounds, then still black screen. Try a few more times, with the same results. Then it goes straight to my Lock Screen, ( not the apple turn on screen after shutdown) with the setting widget at bottom of Lock Screen, I click on setting widget, and it explains the emergency button procedure. What is causing my phone to switch off, this is the second time that it has happened in a matter of weeks.

[Re-Titled by Moderator]