The BlackBerry KEYone is known for its tactile buttons. If the button is stuck or "mushy," the device will automatically enter the bootloader every time it restarts. Inspect the side buttons to ensure they aren't physically jammed by debris or a tight-fitting case.
Disclaimer: Some troubleshooting steps, specifically the Auto-Loader, may lead to data loss. blackberry keyone stuck in bootloader menu
The simplest reason a phone stays in the bootloader menu is a temporary logic loop. You can break this loop by forcing a hard reboot. The BlackBerry KEYone is known for its tactile buttons