A polygon 3D model inverts when moved usually because its surface normals—directions that define a face’s front/back—are flipped or not updated correctly.
Normals help 3D software identify visible faces; if reversed, moving the model exposes the "back" face, making it look inverted. This often stems from editing mistakes (like faulty extrusions) or import/export errors that disrupt normal data.
Try using your tool’s "Show Normals" feature (e.g., Blender’s or Maya’s) to check, then use "Recalculate Normals"—this simple step fixes most inversion issues.