The cause is actually known: high amounts of fructose or sugar (which is 50% fructose) -> decrease in nitric oxide production in the liver -> high BP

Like many things, it is diet related. It seems "unknown" because GPs are still lagging well behind modern dietary research and are often nothing more than pill pushers. Lowering your sugar and simple carb intake over the long term, and fixing potentially hidden metabolic syndrome (see skinny fat people with unhealthy livers) is how you reverse it.