Why has the public debt ratio risen so much in OECD countries?

The public debt ratio in OECD countries has risen from 42% of GDP in 1980 to 111% of GDP today. What explanations do we have for this sharp rise in the public debt ratio? The need to replace private debt with public debt during and following recessions? This is not a valid explanation: the private debt ratio has also risen considerably; A constantly expansionary monetary policy, encouraging governments to borrow more? This has been the case since 2010; An increase in demand for risk-free assets from savers? This is a possible explanation from 2010 also; Population ageing, once it sets in? This is possible from 2005; Short-termism on the part of governments? This is a reasonable explanation until 2005, when governments stopped increasing the tax burden to correct previously expansionary fiscal policies.