It is so easy to humiliate the Current Regime, the Mullahs just cannot help themselves. The bullies get so much gratification from this, they must continue to escalate the provocations and taunting to ever higher levels, just to keep the adrenaline flowing. Now that they have won everything they asked for, they are astonished that every time they slap the Current Regime around, nothing much seems to happen.
So they go ahead and slap around Saudi Arabia as well, this is just further insult and teasing. “Yeah, whatcha gonna do about it?”, like Nelson in The Simpsons pointing and laughing “HA ha!”
Fact is, Saudi Arabia is very nearly as helpless as the Current Regime against the Islamic Republic of Iran, as they have a large number of Shi’ites, who would be a sort of fifth column within the Arabian Peninsula, already embedded in their cities and commercial structure. Even their own citizens would not be politically reliable. And the Saudis have no military infrastructure, as they depended for years on hiring the US Army, US Navy, and US Air Force as their mercenaries. The Current Occupant has pretty much ordered the US military to stand down, or if they do engage, it is with a ridiculous set of Rules of Engagement that effectively prevent any serious action at all.
The Saudi "royal" family would be forced to revert to its true nature as part of ISIS. Specifically they are ISIS with a lot more oil bucks. They would simply torture and kill to quash any internal enemies while throwing whatever WMDs they own at the Iranian Shiites. It will be nice watching those two destroy each other since they will also be destroying a lot of the power structure of Islam.