What is the reason for low diastolic pressure and how can it be treated?

Diastolic pressure. Low diastolic pressure relates to the peripheral vascular resistance, the intravascular volume and the integrity of the aortic valve. Any variable that changes one of these factors can relate to low diastolic pressure.
It depends. There are mutliple reasons why the diastolic pressure may be low, often even when the systolic pressure is high. The significance of this varies depending on the cause.