The drop in ranking for China, as your chart illustrates, is mostly because other countries that used to be less corrupt than China, on average, became less corrupt, while China's absolute ranking didn't change much.
In absolute terms, China improved significantly from 2010 (3.5 out of 10) to 2013 (4.0 out of 10), but its ranking fell because other countries worse than it in 2010 improved even more than China did.
Then, it declined in 2014 from 2013 (back to 3.5), however, and that did represent something real. What was starting to be discovered was that became more clear in 2014 and even more clear in 2015 was the economic and transactions were being manipulated to look rosy when they were not in China.
There were already signs of a bubble that the government was covering up in 2012 in the steel industry. International economic bodies were starting to sound the alarm of a bubble that the government was covering up by the summer of 2013.
Economic analysts were noting that Chinese banks had lots of bad debt outstanding that hasn't been written off, artificially inflating GDP by a big amount in October of 2014. About 97% of the profits that manufacturers in China earned in 2014 were from a 60% increase in the value of the investment securities that they held, while their profits from operations rose by only 0.09%. In 2014, around 97% of existing yuan-denominated bonds hold ratings of double-A to triple-A—the best a company can get despite clear evidence of widespread manufacturing industry distress and the bond ratings of almost all publicly listed Chinese companies were wildly overrated. There were numerous anecdotes of a manufacturing bubble in the international news by April of 2015, but those were known to the insiders in the Transparency International study sooner.
In early July of 2015, the Chinese government has basically closed the stock markets for 89% of its publicly traded firms due to a continuing stock market crash; the Shanghai index was then firmly in bear market territory, down 28.6 per cent since the June peak, while the tech-heavy Shenzhen Composite had fallen 33.2 per cent. In response, by late July of 2015, a Chinese government agency, the China Securities Finance Corp (CSF), central bank-backed refinancing institution, came to be "among top 10 shareholders of many listed-firms" as Chinese regulators have stepped in to prop up a collapsing stock market, effectively, turning what had until recently been a mostly theoretical communist basis of the Chinese economy into one in which state ownership of enterprise was again rapidly becoming the norm.