It all boils down to protections for the investors(corporate paymasters) versus protections for labor, environment, and social welfare.
And then there is the matter of dispute resolution, which is done by the panel, not the courts, and the courts have to honor the panel's decision, which means the trade agreement regulations trump national regulatory law.
So the conflict originates with NAFTA where the prez had fast track authority and negotiated the deal in secrecy, after which the Congress could only vote up or down and had no authority to change the agreement.
Because of that problem with NAFTA, in all subsequent trade deals, the Prez has tried to be more transparent in his negotiations on an FTA, which allows Congress to know what is in the FTA prior to the Prez completing the agreement, which allows the Congress some input.
Free mobility of labor.