The moves fit the pattern which the BOJ deems as "excessive market volatility"
USD/JPY fell by nearly 4% in the early morning and that really spooked markets big time. Japan is still on holiday until tomorrow, but surely you can imagine scenes at the BOJ and MOF being like:
Adam posted earlier that we should expect something from the BOJ and I wouldn't be entirely surprised that they would step out of their way to do so. This fits in perfectly with what they normally deem as "excessive market volatility" and they can easily justify intervening to stabilise the currencies market.
Do be reminded that the Japanese central bank intervened in the bond market last year after yields surged higher following the change in their guidance on the bond yields range. The earlier moves have started to calm down now with USD/JPY "only" down by 1.7% currently. However, it's better to be safe than sorry for Japanese authorities.
Even more so as they continue their battle against deflationary pressures in the economy and they can't afford to let the yen run away further or risk otherwise abandoning any hope (not like there was much anyway) of reaching their 2% inflation target.
