Trump criticized Yellen in the past. Now Powell is in the line of fire. The reason is well-known. Trump strongly feels rate hikes are a threat to the growth of the US economy. Some experts agree while others disagree with that stand. Only time will prove whether Trump is right or wrong.
What Powell and Fed did that has angered Trump?
Under Powell, there were in total 4 rate hikes in 2018. (In 2017 there were 3 hikes). The first hike during March revised the rate to 1.50-1.75 range from 1.25-1.50. Then there were two hikes in June and September. The last one in December took the rate to 2.25-2.50. The rate has almost doubled since 2017.
Is Powell-Trump relationship heading in the same direction as that of Trump-Yellen?
The former chairman, Yellen, was not in good terms with Trump (at least towards the end of her first term). She wasn't nominated for the second term. (Historically, it is almost customary for the Fed chiefs to be nominated for the second term.)
Can Trump fire Powell as a last resort? In a recent interview, on a question regarding the President's Criticism, Powell replied he can't be fired because the law doesn't allow that. On a further note, he said he would continue his service in an unbiased manner.