It can be from moving too fast all of a sudden and not enough blood rushes trough you as fast as you move. Not eating well, or maybe lack of sleep, or not drinking enough water and stuff like that.
Sudden change in posture (lying to standing), dehydration, heat, vaso-vagal reactions (seeing something very upsetting)...
i think its from a sudden rush of blood to your brain
high blood pressure can to..