In the news calendar you have already seen events with three stars. Now it's clear why they matter so much: inflation, the rate and US jobs data change the expectations of the whole market at once.
What happens in the first minutes: the price jerks up 1%, then down 2%, then up again. The first move is often false. Brokers widen the spread, because they themselves don't know what the fair price is. Stops fill with slippage.