You have heard about the brain scans done on meditating monks? Seems there's a pretty specific area of the brain that sorts somatic sensory perceptions (received from the body) and external ones (received from eyes, ears, etc.) Meditation supresses the activity in this area (among other somatic changes.) It's theorized this accounts for the "oneness" experience: your brain stops distinguishing between "me and them."
A moment of extreme stress could possibly have a similar effect.