The Senate shall have the sole Power to try all Impeachments. When sitting for that Purpose, they shall be on Oath or Affirmation. When the President of the United States is tried, the Chief Justice shall preside: And no Person shall be convicted without the Concurrence of two thirds of the Members present.

These are the rules set forth by our founders. No Chief-Justice, no valid

Edit: I suspect they are doing this because their constitutional scholar teams are telling them they can't convict a former president in an impeachment trial, so they are saying "fuck it, if it's not valid, let's have someone that's already convinced he's a guilty traitor preside instead of the Chief Justice".