Why do falling prices hurt debtors?
If the borrower is a firm, lower prices means your output is selling for less, so you need to sell more units in order to repay the debt (assuming a constant profit margin).

For an individual, the buried assumption is that wages are also falling in the deflation. In which case, the debt is increasing relative to your wages. However, if your wages have not fallen, falling prices will make it easier for you to repay the debt (you can consume the same amount, and have more money left over to repay debt).

It makes more sense at the macro level, as deflation is normally associated with lower growth and a higher unemployment rate.