I don't see any headlines driving the drop but if anything else, I think we might be seeing some positioning plays in anticipation of the BOJ tomorrow. The Japanese central bank is still the one with the most propensity to surprise markets and I reckon we are seeing some profit-taking in longs considering that Treasury yields are still holding up on the day so far. The pair is now down 0.3% to 147.88 from around 148.30 levels earlier in the session:

USDJPY H1 21-09
USD/JPY vs US Treasury 10-year yields (%) hourly chart