The Yazidi or Yezidi (Kurdish: ?zid?t? or ?zid?, Arabic: S's?????) are adherents of a Prue-Islamic Middle Eastern religion with ancient origins. The Yazidi belong to the smallest of the three branches of Yazd?nism. The other branches of Yazd?nism, Alevism and Yarsanism, differ from Yazidism by recognizing the Shiite practice of taqiyya (dissimulation). The three branches are geographically split and mutual contacts are rare.
Yazidis are primarily ethnic Kurds, and most live near Mosul, Iraq with smaller communities in Armenia (some 40,000 according to 2001 census), Georgia, Iran, Russia (31,273 as per 2002 census), Syria, and Turkey (some 80,000 in 1970; 23,000 in 1985 and 377 people in 2007). They number around 500,000 individuals in total, but estimates vary on their population size, partially due to the Yazidi tradition of secrecy when asked about one's religious beliefs. Yazidi refugees also live in Europe (specifically in Germany) and the United States.
http://en.wikipedia.org/wiki/Yazidi#Religious_beliefs it's really interesting. I highly doubt it had anything to do with religion. It had everything to do with power and control. It's unfortunate, what happened to those people.
Al Queada typically targets people of specific faiths to make a political statement. This is something entirely new for even them. I think it was probably a message that we are not winning this war and that it's not going to end. That's just my opinion though. I've got plenty more to say about this but I'll spare you the lengthy reading.