Sforno on Numbers 5:6/ספורנו על במדבר ה׳:ו׳

1'למעול מעל בה, our traditional sources already told us that the subject is robbery from a proselyte who dies intestate before the robber confessed and made restitution. The robber had committed an act of desecrating the name of G’d in the eyes of the proselyte who must be appalled that a natural born Jew could be guilty of such a deed. This is why the sin of the robber in this verse is described as a “transgression against something sacred.”