These rifles are equipped with a manual safety that absolutely should be on when carrying the rifle with a round in the chamber. Taking off that safety requires a simple flick of the thumb and doesn’t cause any delay in the use of the rifle. There is no excuse to not keep it on.

Unless they messed with it, these rifles are drop safe. Something could have pulled the trigger on the way down.

Edit

From another post below it is very possible that it was a rifle-mounted flashlight that had its switch bumped against and briefly flashed on. That still image really looks like the beam of a flashlight, not a muzzle flash. Especially given how the light is reflected by the snow, not visible in midair in front of the muzzle.