“Hashem commanded us to judge others lkaf zchus.”
1) And that is predicated on the assumption that you’re judging. When you do that, do it favorably.
2) And judging one’s behaviors will then have implications for that person’s Halachic status, and thus you’re judging the person.
3) And the Halachic status of a person – Tzadik, Rosho, or Beinoni – determines your halachic obligation to judge them favorably or not. Thus assuming a previous judgment on person.