Prevent cloth from tearing?

I’m using the latest version of PhysX sdk. I’m playing around with the man-in-the-green-cape sample program. I see something strange.

When I turn off the collision with the character spheres and capsules, the cape behaves as expected. But when I replace the cape obj (ctdm_cape_m.obj) with a different 3d model and lock some of the vertices, the simulation starts off naturally but then the cloth tears (as if by its own weight) and falls off.

Question: How can I disable tearing of cloth? Am I completely mistaken about what I’m observing?