This is first of all because at the zero lower bound falling inflation implies rising real interest rates, causing households to postpone consumption and firms to delay investment, which can then feedback into still lower inflation. In fact, when we took our decision expected real rates had risen by almost half a percentage point in the previous few months alone.
Regarding financial risks from a longer-term perspective, while there is a possibility that prolonged downward pressure on financial institutions' profits may lead to a gradual pullback in financial intermediation, the vulnerability of the financial system could increase, mainly due to the search for yield behavior.