XZ Premium new bugg, phone freezes/unresponsive and restarts itself when I press the switch button

This past month the phone has twice freezed in the middle of browsing the internet,completely unresponsive and when I simply pressed the switch button it automatically restarted itself instead of just closing down the display.

My question is two-folded: Why does it suddenly freeze sporadically (never did that the first month of usage) and why does it respond to the pressing switch button in these moments as a restart command?