If you see something in the corner of your eye it is often movement. That field is specialized to detect movement and also the pathway through your brain is different and faster than your normal visual route. The idea is that it should protect you from danger.

Sometimes when the brain for some reason is a bit on overdrive, what is in the corner of your eye can be detected as moving even if it isn't. That is usually because of stress or massive usage of visuals in general. It can however be a symptom of true hallucinations but that is less common.