Something got screwed up with your link, Tom - was this it?

As for possible motives, these are probably sufficient for the unstable minds of extremists:

... al-Qaida has long opposed the Saudi royal family, accusing it of being insufficiently Islamic and too close to the West, particularly the United States.

By targeting foreigners' housing compounds, the attackers target the backbone of the Saudi economy. Saudi Arabia is home to 6 million expatriate workers, including about 35,000 Americans and 30,000 Britons. The kingdom relies on foreigners in its oil industry, security forces and health sector.

Saudi authorities, under U.S. pressure to act against terrorism and extremism, have been cracking down on homegrown militants since the May 12 Riyadh attacks.