What causes blood pressure to drop to84 62 one my normal blood pressure is between 139 140/8085 I just had major back surgery and it's been really low?

Varies. There are multiple potential causes of low blood pressure in a post surgical patient. I would sure let your doctor know. Potential causes could include a low blood count, a reaction to your medication, heart failure or even a heart attack. Again, I would sure let your doctor know.