On January 3, Saudi Arabia announced that it had executed 47 people. Among them were prominent Shiite cleric and protester Nimr al-Nimr. As Rob Prince wrote at Focal Points, it may have been an attempt at goading Iran, ever growing in arch-enemy-hood in the eyes of Saudi Arabia, into responding and thus somehow torpedo-ing the recent nuclear deal it had signed.
Since King Salman succeeded King Abdullah, Saudi Arabia has been harsher internally and more aggressive externally, e.g., its war on the Houthis in Yemen, which is seen as a way of rolling back Iranian influence. The United States has long maintained a close relationship with Saudi Arabia because of oil, profits from arms sales, the intelligence the Saudis share on Islamic extremists, and Saudi support for the Israeli-Palestinian peace process. The last has been a non-starter recently, though, because, as Richard Sokolsky writes in Foreign Affairs (behind a paywall):
… Saudi support for the Israeli-Palestinian peace process was once seen as critical to achieving a two-state solution. But the peace process is on life support at best, and Saudi Arabia has recently embraced Hamas, a group that is still on the U.S. terrorist list. Faint hopes for a two-state solution tomorrow are no reason to coddle Saudi Arabia today.
Or as the Robin Wright writes at the New Yorker:
The last break in relations between Saudi Arabia and Iran, also initiated by Riyadh, occurred in 1988. It lasted three years. The current split mirrors a fundamental ideological and strategic division across the Middle East that is now at least as significant as the Arab-Israeli divide, which defined Mideast conflicts over the past six decades. The escalating sectarian rift in recent years is also one of the deepest fractures since the original schism between Sunnis and Shiites, nearly fourteen centuries ago, shortly after the death of the Prophet Muhammad. Then there are the ethnic tensions, dating back centuries, between Arabs and Persians. Even if the United States and others succeed in limiting the damage of the immediate crisis, prospects for healing the deeper divide seem more than unlikely.