Im no economist but with negative inflation it drives consumers to put off that next purchase.
Why buy the car you have been saving for, or put a bid on a house, upgrade your TV or get replacement running shoes if prices are staying the same or potentially going down? A way to make sure that people spend now and dont put it off is that they know the longer they wait the higher chance that the shoes are going to be $5 more in a months time or two.
If people arent spending then conversely people arent selling, building, labouring, etc and then everything grinds to a halt. People get laid off, people cant pay rent or mortgages, things go to shit.

At least this is the way i understand it, and like i said im no expert.