There are many possible causes for this increase. Among the more important possible causes are:
- Hard economic times, include growing inequality. When people are having hard economic times, they are more likely to look for scapegoats. This is particularly true when they can see others getting richer while they themselves do not.
- Rising percentage of minorities in the US population. As America becomes less white, some whites feel more threatened. They feel as if they are losing their dominant position in the society. This can lead them to try to fight back via hate groups.
- The election of President Obama. The election of an African-American president serves to reinforce this idea in the minds of some. This is particularly true since Obama's policies are contrary to what many of these people want.
- The 9/11 attacks and the War on Terror. With the focus on terrorism, there is more of a sense that specific kinds of foreigners are out to get us. This can feed into a desire to oppose people who are not "like us".
All of these factors help to account for increases in hate group activity.
Hate groups feed on economic distress and the insecurity that this distress generates. The 'case study' of this is perhaps the rsie of the Nazis in the Germany of the '30s. Hitler's could be said to be standing on a stack of worthless reichsmarks from the hyperinflation that plagued Germany at that time.
In our own time, we have the failing Euro-zone, bank and Wall Street bailouts in the US, and outsourcing of jobs from higher labor costs countries to lower, and -- it is a big AND - social media and instant communication. The hateful screed of Hitler, for example, either had to be purchased - as in Mein Kampf -- or listened to in his (initially) small meetings. Today, technology is the silent abettor of hate.