[kqr]

This is in some sense also the primary cause of inflation in many cases. When demand for money drops, its value relative to stuff decreases, and you need more of it for the same things. (This is often followed by an increase in money supply, not, primarily, caused by it. The increase in supply is usually people (government or their ilk) creating more money to afford things once it's value has dropped.)

So why does demand for money go down in the first place? Now that's complicated. The demand for money is determined by the market which is sometimes predictable and sometimes works in mysterious ways.