US oil production increased considerably over the past few years, then the Saudis deliberately increased production to drive down the high prices that had in part enabled the increased US production.

"Saudi Arabia has maintained a high level of oil production despite the slump in crude oil prices, arguing it needs to protect its market share in an economy in flux because of the glut of U.S. oil."

http://www.upi.com/Business_News/Energy-Resources/2015/07/13...