The Saudis are mad because of Russian and Iranian support for legitimate President of Syria, Bashar Assad. They know that dropping the price of oil and forcing a global glut hurts both of these rivals' economies. They wish to hurt them and prepare them for Western sponsored regime change.
If the ayatollahs are overthrown it will be the best day for the Saudi monarchy since they found the black gold.
The US is producing more oil to do the same. On the one hand they want to gain market share over Saudi Arabia, but they also see the benefits of weakening the nations that dare stand up to them.
It is true that in an economic sense Saudis and USA are rivals but they are both gleeful frenemies who are all to happy to see Russia and Iran killed in the crossfire.