Some of the causes for hypotension are sepsis, dehydration, effect of
antihypertensive drugs, pulmonary embolism, heart attack, bradycardia or tachycardia etc.
As, mentioned your father has a history which can be a risk factor for developing recurrent infections or inflammation of the heart muscle or might be due to the drugs he is on.
I would recommend that he should get evaluated for the above mentioned causes for further confirmation on the cause for this. Best.