Bad CPI number means less inflationary pressure, and this eases the FED worries regarding inflation that might prevent them from further rate cuts. This usually gives a positive sign for the equity market, and the stocks start to rally, increasing the demand for the US dollar (to buy stocks), which usually strengthens U/J, and this will of course strengthens the beast (G/J) by pushing the value of the Yen down.
And we did have a bad cpi on friday.