I think there's a worry about some Al Queda / ISIS types taking over and causing havoc with the Saudi trillions. The Saudis may be undemocratic but at least they don't cause too much trouble.
But Saudi Arabia funded/funds Al-Qaeda and ISIS, they are Sunni terror groups, despite the fact that in theory, Saudi Arabia is a US ally. These groups attack Shias and other minority groups, not other Sunnis like the Saudis. Remember, a lot of the 9/11 hijackers were Saudis, and a lot of the funding came from Saudi Arabia. The US is funding its enemies by its strong links to Saudi Arabia.
You can read this article, or google around for better ones:
It's a very complicated relationship, I think the best long term outcome for the region and the world is a massive reduction in the importance of oil and the Middle East ceases to be important strategically.
They exported and funded their errant knights using those trillions. The main reason the Saudi government has been left alone by the more extreme elements in Saudi Arabia is that they are the main conduit for creating petrodollars, so if they were disrupted, the money hose might stop.
