991 Saudis have been simultaneously indicted for terrorism (technically, "acts of rebellion" or "acts of war" against the state) over the past 5 years by the Riyadh government. Recall first that many Saudis were involved in the 9/11 attacks, and that foreigners in Saudi Arabia have been attacked with bothersome frequency by Wahhabi terrorists. Americans mostly perceive the Saud royal family of spending more time bathing in petrodollars than doing anything to help, despite being an alleged "friend" of the Bush regime.
It's a bit different than that. The Saudi government doesn't really like the Wahhabis or Al-Qaeda or other fundamentalist nutjob terrorists, despite their own fundamentalist leanings, and despite their support of very questionable Sunni militants in Iraq. Al-Qaeda and the Wahhabis don't really like Saudi royal rule, and often make trouble for the population. Terrorism in Saudi Arabia has killed 160 citizens and wounded hundreds more.
Why hadn't the Sauds dealt with it yet? Part of it is that they have a surprisingly tenuous grasp on power. They're not about to fall, but they don't have much in the way of representative legitimacy, and Wahhabism is pretty darn powerful in Saudi Arabia. The Saud government certainly wanted to avoid an all-out war with its own Wahhab people; it was happier to leave them alone.
But why now? Well, the unhappiness by the Sauds with the Wahhabis didn't go away--the Saud family was not thrilled to have them around. The huge number of simultaneous indictments indicates that this was a "one swift stroke" kind of motion--the Saud government is making a power grab. It's a risky move--if it's not intimidating enough, Wahhabis may rebel en masse--creating a messy internal war (much like the one that Pakistan is trying to fight now). But if the show of force is sufficient--if it can disrupt, demoralize, and decapitate the Al-Qaeda and Wahhabi organization, then they've got a good shot at nearly permanently weakening their extremist groups.
The timing may also have to do with Iraq--they may have decided that the civil war is over, and they no longer need to stay friendly with their own Sunni insurgents/extremists to keep funding the militias in Iraq. The Iraqi factor may have been holding them back for the last 5 years, and may finally be over.