tl;dr The Whitehouse is accusing the EPA of nisconduct, so it sent a letter to Citibank to sieze funds the EPA had sent to Citibank to distribute to fulfill contracts to green power initiatives. The Federal Prosecutors resigned rather than send that letter based on no evidence.
So they want to retract their guilty plea so they only have to pay fines for hundreds of deaths. And Trump will probably direct the DOJ to comply. Because Justice is only for the wealthy now.