Why were Michael the Archangel and Lucifer arguing over the body of Moses when the old patriarch already was with the Lord?

 The story you are referring to is not explicitly detailed in the canonical books of the Bible but is found in apocryphal or extra-biblical works, specifically in the "Assumption of Moses" or the "Testament of Moses." This text is considered part of Jewish pseudepigrapha and is not included in the standard biblical canon.

In the apocryphal account, the dispute over the body of Moses is described as a confrontation between Michael the Archangel and Satan (often identified with Lucifer or a rebellious angel). According to the narrative, Moses dies, and there is a disagreement between Michael and Satan regarding the disposition of Moses' body.

The exact reasons for this dispute vary in different versions of the story. Some interpretations suggest that Satan may have accused Moses of murder, as Moses killed an Egyptian earlier in his life. Others propose that Satan sought to prevent Moses' body from receiving an honorable burial, which was traditionally significant in Jewish culture.

In any case, Michael, described as the defender of the righteous, ultimately prevails, and Moses' body is taken away from Satan. The story does not provide explicit details about the exact location or nature of Moses' burial, as the emphasis is on the celestial struggle over his body.

It's important to note that interpretations of this narrative may vary among different religious traditions, and not all groups accept the "Assumption of Moses" or similar apocryphal texts as authoritative. The canonical books of the Bible, particularly the Torah (the first five books), do not contain this specific account of the dispute over Moses' body.