An inverse relationship exists between inflation and joblessness because an unexpected increase in inflation pushes down real wages, lowers the price of labour, and thus tends to lead to a drop in unemployment. But that only happens in the short run.
Because employees will push for the higher rate of inflation to be offset, thus moving real wages and unemployment back to where they were before. There is a shift in the Phillips curve. And if the unions, fearing a further increase in the rate of inflation, push through even higher wage increases, unemployment will rise as a result.