I don't think it does have a single simple cause.
You could argue:
- It was the French colonizing the country and imposing a nasty explotative regime
- The French for refusing to decolonize the country post WW2 and fightning (and losing) a war to retain their colony
- The US for refusing to help Ho Chi Minh when asked
- The US for helping the French
- The corrupt and brutal South Vietnamese regime propped up by the US
- The brutal North Vietnam regime encouraged by the Soviets and Chinese to fight a proxy war against the West
- The US for escalating after the Gulf of Tonkin incident