I agree with most of what has been said in the posts above, particularly that the hippie movement was part of a "perfect storm" of desire for social and political reform following the end of WWII and the materialistic 1950s. One of the things that led to the movement was the fact that the 50s seemed like a prosperous time and yet racism was still rampant, making it hypocritical to say this was a "good time" in American history. I think, out of that complacency, a frustration emerged from a growing dissatisfaction with sluggish social progress even in the wake of increasing technological process. So, some people in America were benefiting during the decades running up to the hippie movement, but many weren't. This is a recipe for a social movement/revolution.
I would only add that the anti-war movement was incredibly significant, as it has already been said. Remember that there was a draft, a mandatory enlistment to fight in the war. The hippie movement raged against this idea of the government forcing young people to fight in a war they felt was totally unjustified. So, the movement was against the war which was American imperialism abroad, but also...
domestically (McCarthyism occurring in the 1950s).
The hippie movement gets a bad rap sometimes because, to some, it was an escape, a happy-go-lucky lifestyle. It was that for some, but many hippies were aligned with groups like the Black Panthers and the SDS (Students for a Democratic Society), militant groups who were very serious about instigating social reforms.