This mixes up the reason and the cause. A deflationary spiral occurs when you have a financial crisis, e.g. due to a debt-based asset bubble, and the economy is not strong enough to stimulate growth through policy changes, e.g. interest rates are already zero etc. This then in turn leads to reduced spending to ensure liquidity, that is reduced demand, which leads to lower prices, which is deflation. Deflation is the result, not the cause. From that point onward, yes, it is possible for the situation to spiral in a feedback loop, but deflation is not why the spiral started.