Orthostatic hypertension—a rise in blood pressure upon assuming upright posture—is an underappreciated and understudied clinical phenomenon. There is currently no widely agreed-upon definition of clinical orthostatic hypertension, the current definitions being operational within the context of particular studies. The underlying pathophysiology is thought to involve activation of the sympathetic nervous system, but the actual etiology is poorly understood.

Source : Nature Clinical Practice Nephrology volume 2, pages 424–431

Since the research paper above was written (2006) an international expert panel have published a proposal for a definition :

Orthostatic hypertension was defined as an exaggerated orthostatic pressor response associated with systolic blood pressure of at least 140 mmHg while standing."

ref.Blood Pressure, Volume 32, 2023 - Issue 1

New research estimates that somewhere between 3% and 30% of adults might have a measurable degree of orthostatic hypertension (previous estimates were around 1%)

It's proposed that the syndrome might be caused by "sympathetic activation overshoot" - viz. an overcompensation may happen when the Sympathetic Nervous System tries to compensate for a gravity-induced lowering of blood pressure around the brain as a person stands up. But there is still no formally agreed explanation of the syndrome.

Given its apparent ubiquity, it seems the syndrome doesn't pose much of a threat to healthy individuals. But those with other pre-existing cardiac and/or blood-pressure problems may be at risk - though it's not known to what extent. To date there are currently no officially recommended treatments.

Also see : Hypertensionplugin-autotooltip__plain plugin-autotooltip_bigHypertension

Only 5 -10% of cases of hypertension (high blood pressure) have causes which can be clearly identified - e.g. due to narrowing of the arteries. Put another way, of those patients who have been identified as having abnormally high blood pressure, physicians will not be able to pinpoint a specific cause in more than 90% of cases.