This laser driver board seems to be common for many <20W LED lasers used for cutting/engraving.
Whenever powered on, it defaults to full power due to a rather strong internal pull-up.
Most CNC machines have a laser TTL PWM output that goes into high-impedance mode on boot. In that case, any connection to, or reboot of the microcontroller will make the laser output full power for about 2 seconds while the user may not be prepared for that.
The solution is to remove D3, which provides power to the pull-up, then the laser will not be “default on” anymore.