Modifiers cause 3D models to stretch mainly due to poor base topology (e.g., triangles, uneven edge loops) or incorrect modifier settings (like misaligned UVs, unbalanced weight painting).
Modifiers rely on the mesh’s underlying structure—flaws like jagged polygons or unbalanced weight maps get amplified, pulling vertices unevenly. For example, a Subdivision Surface modifier might stretch a triangular face, while an Armature modifier could distort a limb if weight painting doesn’t follow the model’s shape.
To fix it, clean your base mesh to use mostly quads, then adjust modifier settings (e.g., fix UV seams or smooth weight maps)—these simple steps often resolve the issue.