There are two possible factors; the first is more likely. As light levels fall, the pupils open wider so that the light from the flash hits the retina behind the eyeball. This is what causes red-eye, flash-eye, steel eye, whatever you wish to call it.

The only way to prevent it is to get the flash well off of the lens with a long cord or a radio trigger.

The second phenomena is much rarer and is caused by a very low angled sun; in these cases the sun hits the retina and causes sever red-eye. This only happens when the sun is just above the horizon. I have only seen it two or three times in 32+ years. Red-eye without flash!

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Yes, using a flash bracket that has the flash seven inches above the enter of the lens. Went back and looked at the images with the eye problem and it appears all were made at the edge of light…before sunrise and after sunset. Looking forward to the answer.