In a nutshell, there has been a continuous dispute between Japan and China over several unclaimed islands in South East Asia. Fundamentally, it all comes down to oil, mining, fishing, and other natural resources that are potentially on these islands. Here is an easy and accessible graph that sums it up:

The US are (unsurprisingly) getting involved in this dispute, so China have taken a naturally aggressive stance. It will be interesting if Russia, South and North Korea starting getting involved, as well.