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.