Why?
I hypothesize that it results from combining the worsts of capitalism and communism, with historical ties. Currently, China's economy is capitalist with no safety nets. Its politics are communist with no other parties or elections allowed. Chinese experience the "capitalist anxiety" where they focus on wealth accumulation and live in fear of anything that could wrong, taking wealth away. They also experience the "communist anxiety", always wondering if what they have accumulated will be nationalized.
The "communist anxiety" is a remnant from the Cultural Revolution (1966-1976) where landlords were overthrown and all the country's land were collectivized. That Revolution morphed into State control of everything, where one's life was not her own choosing. For example, nearly everyone with a high school degree were sent to countrysides to be 'reeducated and reformed'. Merit-based education was banned throughout the country, taken over by propaganda-based education. In the countryside, it was illegal to raise your own chickens because they were not collective property. In 2009's market economy, where everything (economically, not politically) is to your own choosing, there is no wonder that people are so anxious to safeguard possessions against anything that might happen.
Here, my Marxist friend counters that it's not anxiety I'm describing, but materialism. People can choose non-materialistic pursuits. Yes, but choosing a path that is not the one that everyone else is doing requires critical thinking: the exact thing oppressed in schools and society. Seeing their hold on power cannot be sustained from isolation like North Korea's, the Chinese communist government opened its doors to capitalism in order to draw people's attention to material gains rather than rights and freedoms. With people focused on getting Mercedes, they have no interest in participating in social movements. Hence while it is possible to critically examine one's life in spite of hegemony, it is very, very hard.
On top of these historical, economic, political, social conditions, add in a long-standing culture valuing achievement and work ethic, and the Chinese become the most anxious peoples.
There is a deep psychological price to be paid in being the 'Economic Miracle' and 'Model Minority'.