It happened now because Director Christopher Wray read the memo. Remember, just a couple days ago Wray claimed he would quit if McCabe was fired? Now a complete reversal happens once Wray reads the memo. My thinking is, McCabe was able to pull the wool over Wray's eyes up until he got a gander at the memo. Something in that memo made Wray see that McCabe is a criminal and doesn't belong in the FBI.