The rays of the sun bounce off the lens of the camera, projecting a reddish glow into the eyes of those photographed.
An ineffective flash doesn't allow enough light for the actual eye color to be recorded in the photo.
A flash that is too glaring often makes people blink at the same time the shutter snaps. Therefore, the subjects eyes are merely blurred, though it appears red in the actual photo.
The pupil doesn't have enough time to constrict and the picture records the reflection of the red retina in the back of the eye.