The Washington Institute interprets the targeting of Shiites in Saudi Arabia as hitting at a vulnerable point in their religio-political ideology:
Over the past two weeks, the so-called “Islamic State” (IS) has claimed two attacks on Shiite mosques in Saudi Arabia’s Shiite-majority Eastern Province, one in Dammam and the other in Qatif. While the incidents might not have an immediate impact on the kingdom’s overall security, they are relevant to long-term IS strategy of weakening the Saudi government by exposing its alleged hypocrisy.
A nation-state is home to all its citizens. But …
By attacking the Eastern Province, IS seeks to place Riyadh in the position of defending or appeasing Shiites, at the expense of a Saudi Wahhabist state ideology that does not tread too far from that of IS (e.g., Saudi schools teach students that Shiites are unbelievers and not Muslims).
The article describes how official government response has been to condemn the attack and offer condolences to its victims. By international standards this is the absolute minimum requirement. Not mentioned is the bounty Saudi Arabia offered for information leading to the criminals.
But The Islamic State is not interested in the international standards:
From the Islamic State’s perspective, such actions highlight Riyadh’s rank hypocrisy, showing “true” believers in the “land of the two holy places” how the Saudi state is contravening both God and its own founding standards. By casting themselves as the true bearers of Islam, IS leaders hope to draw more recruits and supporters.
The Saudi government is in a tricky spot. A long time ago they made a deal with you know who. Is it now coming due?