In a "balanced sheet recession", aggregate demand falls

because businesses become pessimistic about future rates of return and stop investing.
because government creates fiscal drag by running budget surpluses.
because businesses and households start saving to reduce the debt exposure they created in the boom.

Answer: because businesses and households start saving to reduce the debt exposure they created in the boom.

Please read Balance sheet recessions and democracy for more information or post a comment if you are unsure.