The problem arises from the fact that, when inflation is high and nominal interest rates are similarly high, the cash-flow problem of servicing a loan is quite difficult in the first few years of the loan, but very easy in the last few years of the loan.
Put another way, using interest rates to compensate savers for the effects of inflation on their savings has the effect of front-loading the real burden of debt service. This may effectively deny those on low incomes any access to borrowing facilities in times of high inflation, even though the real interest rate on the loan is at a moderate level.