
China's rapid debt rise started in 2008.

2008-10: 4 Trillion Plan (Chapter 3); 2010-11: inflation (pork prices rise), adjustment (more restrictive monetary policy); 2011-15: European debt crisis, more stimulation (lower interest rates, looser lending requirements for local governments), shadow banking; 2015-16: stock market crash, capital flight (RMB devaluation), debt swap for wealth management products explode; 2016-18: "live not speculate" housing policy, decrease capacity, new asset management rules
2018 Total Debt vs. GDP

China has less government debt than developed countries, but more corporate debt (154% in China vs 75% in US and 58% in Germany). This is because equity capital markets are underdeveloped & the percentage of equity vs. debt fundraising is low (total equity markets just ~7Trn RMB).
There's also the rapid growth of SOEs vs private companies. After 2008, SOEs grew a lot but less efficiently than private companies, so the capital they took up did not translate to corresponding increases in income. From 1998-2007, SOE assets only grew 1.6x, but from 2008-2017, 4.4x (debt increased 4.7x, and grew from 78% to 144% of GDP). Profits as % of GDP decreased from 4.2% to 3.9%, revenues from 72% to 65%.
Real Estate industry debt is also a problem, partly because it drives so much of GDP growth (30% in 2013) and accounts for so much of GDP (30 in 2013). In developed countries, developers cannot borrow money to buy land, but in China, they can. By 2018, debt from the real estate industry was 75% of GDP, and lots from shadow banking & other unregulated channels. And pre-sales and personal mortgages account for 50% of cash on hand. But as soon as housing sales decline, this entire capital chain can break down, such as we see with Evergrande. And if real estate companies start having debt crises, it will also hit the financial system and the economy as a whole. Since local governments also rely on selling land for their revenues, this also affects their credit. Thus in August 2020, the central government put out rules on how real estate companies cannot get debt to buy land.
A final point is foreign debt. These real estate companies have no foreign revenue but $174Bn in foreign currency debt. That is also a risk & the NDRC has restricted this since 2019.
Overall, Chinese companies have a heavy debt burden and limited ability to deal with risk. If there is a crisis, then there is risk of default.