Agents might tend to have such expectations if they foresee the economy remaining in recession for a protracted period or if they project deflationary pressures ahead. This would be reflected in a flat yield curve and low long-term rates.
However, if the yield curve does not reflect such deflationary expectations, the central bank may try to flatten it by committing to maintain low interest rates for a prolonged period of time, and may try to convince agents that they can safely invest their savings in long-term assets without incurring in capital losses. Yet such a policy may encounter a series of additional problems.