When you suffer from high diastolic blood pressure, an abnormal amount of force is being placed on your arteries when your heart is at rest. Normally, your diastolic blood pressure (the bottom number in your blood pressure reading) should be below 80 mm Hg (millimeters of mercury). If it is above this number, it can be due to a number of contributing factors.

Age

One of the main culprits of high diastolic blood pressure is age. As you get older, you're at a greater risk of developing greater pressure in your blood when you're heart beats and is at rest.

Weight

Another common factor for high diastolic blood pressure is your weight. As you carry an increasing amount of weight, your blood must travel that much greater of distance, forcing your heart to work harder and causing an increase in both systolic (top number) and diastolic blood pressure.

Inactivity

Often going hand-in-hand with weight, people living a more sedentary lifestyle tend to experience an increase in the beating of their hearts, which impacts the force of blood in their arteries.

Diet

Diet can also contribute to the development of high diastolic blood pressure. Eating habits high in sodium and fat as well as low in potassium and vitamin D can increase diastolic blood pressure.

Smoking

Cigarette smoke can damage the elasticity of your blood vessels, causing them to harden and narrow. With this reduced room for blood to flow, the pressure increases, resulting in a higher diastolic (and systolic) blood pressure.

Other Conditions

It's also possible to experience an elevation in diastolic blood pressure from a preexisting condition, such as diabetes, high cholesterol and kidney disorders.

Source:

Mayo Clinic: High Blood Pressure

Cleveland Clinic: High Blood Pressure