That's one facet another is how deflation and loans mix. In effect deflation increases interest rates just as inflation decreases them. If you owe 1$ @ 5% over a year with 50% deflation you now owe 2.1 times the value of what you bought. Once deflation starts people start trying to pay back their debt because their effective interest rate increased. Which decreases the money supply and causes even more deflation. This cycle can quickly get extremely nasty with most people defaulting.

The free market dampener is people refinancing at slightly lower rates, but this this hits a wall. Negative interest rates never happens, because why lend money when you can just keep it. And even mild deflation is hard to deal with because loans have risk so you can't lower loans below the rate of risk.