What reason did the House give for impeaching President Andrew Johnson?

The ostensible reason the Radical Republicans wanted to impeach President Johnson was because he attempted to fire Secretary of War Edwin M. Stanton over disagreements about Reconstruction. Stanton was supposed to be protected by the Tenure of Office Act Congress passed in 1867, that prevented the President from removing from office anyone appointed with and by the "advice and consent" of the Senate. Johnson considered the law unconstitutional and ignored it.

The US Supreme Court eventually agreed with Johnson in Myers v. United States, 272 US 52 (1926) when the Taft Court held the President has exclusive power to remove executive branch officials without the consent of the Senate or any other legislative body. In retrospect, the Court determined the impeachment proceedings against Johnson constitutionally invalid, but that was not part of the holding in Myers.