Older patients are more prone to a decrease in blood pressure upon standing because the body is less able to adapt the BP to rapid postural changes.
When a patient standing, some of the blood come to the leg. With less blood, the heart is harder to maintain its pressure.
The body will respond with constricting the vessel or increasing heart muscle power. But in an elderly patient, this response is weaker. Some of the older patients also consume a drug that will disturb this mechanism.