For syncope/black outs to occur, either the reticular activating system in the brain needs to lose its
blood supply, or both hemispheres of the brain need to be deprived of
blood, oxygen, or glucose.
So it can be due to fall in BP, hypoglycemia, dehydration and inadequate electrolytes(sodium and calcium) in the body, postural hypotension, blockage of arteries supplying blood to the brain esp. the carotids,anemia,vasovagal attacks or underlying neurological conditions.