First, when policy interest rates are at zero – as was the case by that time – falling inflation expectations imply a rise in real interest rates, which amounts to a tightening of our monetary policy stance. If left unaddressed, that could have started a further fall in inflation expectations and a sharper tightening of the monetary stance.
Second, though oil price movements can permanently affect the price level, they should not exert a longer-term influence on the inflation rate. So the fact that we were seeing longerterm inflation expectations responding to low oil prices was a source of concern.