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.
Let me use an everyday situation to shed more light on how this principle works. Imagine a person who is habitually late for work. Now, their partner might be able to outsmart them once by moving the hands of the kitchen clock forward by five minutes.