What can cause a high systolic and low diastolic blood pressure?

Aging.: As we age vessels become stiffer. Stiffer vessels are associated with this.

Many causes: Dr. Fisher correctly notes that the most common causes is stiffening of the vessels as we age. Widened pulse pressure (the difference) is common if you're simply excited, athletic, have a fever, are hyperthyroid, have paget's of the bone or a dialysis shunt, are anemic or pregnant, or have an aortic valve that won't close right.