Probably dehydration or too few calories. Could also be a vasovagal response which is a rapid drop in blood pressure due to increased intraabdominal pressure. The increased abdominal pressure decreases cardiac output causing the drop in blood pressure and subsequent lack of oxygen to your brain. Then comes the tunnel vision and if you don’t finish the lift quickly enough, it’s lights out. We’ve had a few guys at CFES have that happen while grinding through a heavy deadlift.