Do you know that newborns can scare themselves?
It is not necessary that happens something special to startle a newborn: bright light, sudden motion, loud noises, even their own cries, especially when they cry aloud.
You will know that your newborn has scared because he flings his arms out to the side with the hands open, and then quickly closes and tucks back in toward his body.
Although you can’t know when it is going to happen, you can calm him after, with sweet “SSHH” sounds, and gentle touches.