He's doing this now because he clearly thinks this is how to corrupt the economy for political gain. And Trumpy wants in on dat! I'm sure it went like this with the Fed Chair:
"Obama had his people do this for him and you better do it for me too because he's a loser and I'm tough. Also strong. I also have good genes. And there's NO PROBLEM down there! Believe me. Believe me."
"Theyre keeping the rates artificially low so the economy doesnt go down," Trump said in response to a question about a potential rate hike by the Federal Reserve this month. "So that Obama can say he did a good job. That's the only reason that the rates are so low. They're keeping the rates artificially low so that Obama can go out and play golf after January and say that he did a good job. But it is a very, a very false economy. So [Obama] can leave office and say: See I told you."
We have a very false economy," he repeated.
"So far, I think she's done a political job," he said about Fed chair Janet Yellen.
"At some point the rates are going to have to change," Trump added. "The only thing that is strong is the artificial stock market."
https://www.realclearpolitics.com/video/2016/09/06/trump_the_fed_keeping_interest_rates_artificially_low_so_the_economy_doesnt_go_down.html