What explains the rise in the total debt ratio in OECD countries since 1999?

The total debt ratio in OECD countries has risen sharply since 1999. This makes OECD economies more fragile and forces monetary policy to remain expansionary. What accounts for this shift towards borrowing in OECD countries? The possible explanations are: The slowdown in household demand due to the skewing of income distribution against wage earners; The decline in potential growth , which governments in OECD countries have mistakenly combated with demand-stimulus policies; The shift to a more expansionary monetary policy, leading to a higher optimal debt ratio, especially as an expansionary monetary policy increases wealth.