Deflation can be devastating to those in debt. If you take out a loan for a house or a tractor, then subsequently discovering that you have to pay back substantially more than you borrowed (before even factoring in interest!) could be quite a shock.

To give a simplified example, if a farmer takes out a $50k loan, then at current prices they might be thinking that can be repaid by selling 10,000 bushels of produce at $5/bushel. However, if there is subsequently 20% deflation, then the price would fall to $4/bushel and the farmer would all of a sudden find they have to sell an additional 2500 bushels to get out of debt. If they can't, they risk losing everything to foreclosure.