They worry that, in a period of very low inflation, the Bank may do things with monetary policy, in an effort to increase inflation back to the target in short order, that might create more problems than they solve.
BIS central bankers’ speeches  7  I can assure you that the Board has been very conscious of that possibility and, accordingly, has proceeded very carefully. Of course we have run some risks from pushing interest rates so low, but then there are always risks in any course of action, including inaction.