My Chinese coworker has told me that this usually only occurs during the massive traffic jams around the spring festival. He said that many Chinese people who live in cities own cars but almost never drive them -- they wanted the car more for status than anything. So when spring festival comes around and the whole country is traveling, many families see it as their chance to finally actually use the car. Additionally, apparently the major highways -- their version of interstates -- are toll roads, but are made free during that week.

So both of these factors, combined simply with the massive volume, is what leads to these apocalyptic traffic jams we've all seen pictures of.