The irony of this situation is that Sikhism is generally known as the warrior religion, they have been persecuted since the 15th century when Mughals (muslims) ruled over India. They gave up their lives (read about the ten gurus) because they would not convert to Islam. The founder, guru Nanak, walked from india to Iraq preaching equality of the human race, which eventually transformed into a new religion (Sikhism). It was a subculture of India that went against social Norms, like the caste system and injustice to females (which included, but not limited to, throwing the wife on the cremating body of the husband because that was her "duty". The reason most Sikhs have "Singh" as their last name is because it omitted any correlation to the caste system, as last names indicated your caste. Singh comes from Simha, a Sanskrit word for lion - it suited Sikhs because they had a "natural mane", i.e unshorn hair, and fearless mentalities like that of a lion. This was adopted by the tenth guru, Gobind Singh, who created an equal brotherhood to defend humanity, mainly from religious persecution. He himself was an aristocrat who lost his 4 sons to the Mughals, who confined the youngest son in a brick wall for not converting into Islam. His mother and middle son were locked on a tall tower, where they suffered from the elements, why? Because they didn't want to convert to islam. And eventually his whole family was killed and he created the "Khalsa - or Sikh brotherhood". So, a religion was officially created and centuries go on, with a little bickering here and there.
They number into the millions and in 1984 another dent is put into the community when Indira Ghandi, Indian prime minister, sent troops to storm the Golden Temple, because separatists wanted their own Sikh nation, where Sikhism would be recognized and not just "another branch of Hinduism" - which I should mention was notarized by the predominant Hindu government of India. So her Sikh bodyguards backfire, literally, by assassinating her. This led to nation-wide anti-Sikh riots, (now keep in mind, that only a small state, Punjab, is predominately Sikh). Sikh were burned alive and Sikh's homes/businesses were burned to the ground by Hindu mobsters. The Indian government had a two-day media black-out, so the numbers of Sikhs killed is not official. But the India government has reported about 10-20,000 deaths. A record kept by a human-rights defender added another several thousands to the "official-count", meaning that anywhere from
50,000-100,000 Sikhs were killed or went missing from 1984-1990 due to government-organized attacks. So due to this reason, many Sikhs fled to Canada, America, England, Singapore and other nations alike, seeking political-asylum, hence the large amount of Sikhs around the world (well, this added to the fact, but Sikhs have always been persecuted by the India government). 
So, the new millennium starts, and everything seems smooth and then BAM! (no pun intended) 9/11 strikes and hate crimes flood the Sikh community, this was a lot worse for Sikhs than Muslims, because Sikhs looked more foreign, and hence "like arabs", because of their turbans and beard. Which I can see, because if you look at Osama Bin Laden and a Sikh man with a turban, (sorry I think it's kind of cool that a guy with a turban plays in the NFL), it's hard to tell the difference, without having previous knowledge. So following this, a Sikh gas station owner in Arizona was shot and killed by some lunatic who thought he was being a "patriot" and killing Muslims (wrong religion, but a wrong motive overall).. And now this happens. What I don't understand is that a group, whose motive has been to abolish the caste system and create equality; ones that "open all-doors" to their temples because a belief is embedded that God is not particular to a group but to humanity overall are targeted because people do not choose to learn about who they are. A simple Google search of "who are the guys that mostly wear turbans, or things on their head,etc..." reveals that Sikhs are the predominate race, especially in the West, that wear turbans and keep beards. This creates angerbut not against "white people" or Americans, but against the media who ignore the fact that they need to showcase individual identities and not focus on Islam and Christianity. In fact, when Balbir Singh Sodhi was killed, the media reported it as "Arab man shot and killed at Gas station", which was later corrected when the Sikh Coalition reported to different agencies that "look guys, you see a turban, don't do your research and try to get your news corporation's word out first, but it's causing harm to our community".
Source: Am a Sikh, well an atheist, but I choose to keep my hair long for spiritual religions that aren't attached to God and practice yoga and meditation.
P.S: There is a huge community of "Gore Sikh" or White Sikh, that converted in the 1970's, mainly because the Sikh ideology attached to nature and meditation attracted many hippies.
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