Writer Ian Buruma attempts to answer that question in a piece in the Guardian:
China’s recent thuggish behaviour is changing Asian opinions. As the warm welcome given to Hillary Clinton on her recent swing through Asia – even in communist Vietnam – appears to show, south-east Asians are more than happy to hang on to Pax Americana for a bit longer, out of fear of China. Other Asian countries might even be drawn closer to Japan, the only alternative to the US as a counterbalance to the Middle Kingdom. This cannot be what China wants.
So why is China being so severe? One possible explanation is that China is a little drunk on its new great-power status. For the first time in almost 200 years, China can really throw its weight around, and it will do what it wants, regardless of what other countries may think. A few decades ago, it was Japan that thought it was going to be No1, and its businessmen, politicians, and bureaucrats were not shy about letting the rest of the world know. Call China’s recent actions revenge for a century of humiliation by stronger powers.
But this may not be the best explanation for China’s behaviour. In fact, the reason may be just the opposite: a sense among China’s rulers of weakness at home. At least since 1989, the legitimacy of the Chinese Communist party’s monopoly on power has been fragile. Communist ideology is a spent force. Using the People’s Liberation Army to murder civilian protesters, not only in Beijing but all over China in June 1989, further undermined the one-party system’s legitimacy.
The way to regain the support of the burgeoning Chinese middle class was to promise a quick leap to greater prosperity through high-speed economic growth.