If the photographer had set the camera to be operated by a remote control, and clicked the switch, or set up a shot with a lightning sensor, or dropped the camera and it landed on a pebble which triggered the shutter release, few would argue that the photographer wouldn't own the copyright. The monkey might be analogous in the situation.

Ultimately it's just a place where the judge has to have an opinion because the law doesn't really have a plan for such an improbable situation.