I'll take a stab, not much knowledge of the specifics.

Relatively high household and govt debt levels + anaemic growth causing central bank to lag on increasing rates to the level that the market sees as worthy of the risk.

Am I close? It's the AUD model but I figure it's close.

It's all about monetary policy, Japan is also affected by this as their interest rates are out of sync with North America, pushing people to leave the local currency to pursue higher interest rates in US Dollars.