This happened to me. I hadn't been drinking or using drugs, but I had an eating disorder, was underweight and had been crushed at a music concert. I'd gone to join the queue to collect our belongings at the end of the show. As the cloak desk handed over our coats and bags, my eyesight started going, like an old TV screen, not plain black but static black. I saw the assistant put our belongings on the desk before it went completely. Sightless, I reached out, picked them up in a bundle, and went to the barrier opposite, where I'd seen a moment earlier, I could just collapse out of the way. I waited there for a few minutes with the bags on the floor, wondering what the hell to do. Slowly my sight returned.

I think it's when the body is under severe stress and probably malnourished.