Usually when production decreases, cost of production increases - operation becomes less profitable and investor might pull their money and take it elsewhere. In some industries, margin might be so thin that once they lose economies of scale which comes with higher production, they simply may go bust or choose to stop, citing higher cost of inputs. This is what pushes the supply below demand level.
Also demand is usually established at some specific price, if cost of production increases, producer might have to increase the price and this might also put downward pressure on demand.
Inflation has been low for years and has historically gone done during recessions. The inflation hawk concerns so far have been utterly unfounded.
https://www.usinflationcalculator.com/inflation/historical-i...
Edit: It is a completely serious answer and the only explanation that makes sense to me for the spate of pro-UBI articles I've noticed but mostly not read because UBI doesn't make sense and a global pandemic doesn't change that.