For syncope/fainting 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.