For the life of me, why can't I unwrap this object? [blend included]

I’ve spent a good hour now placing seams in every which area that would normally make sense but to no avail. No matter what I do it feels like Blender is just randomly throwing vertices around when I unwrap. Can anyone offer insight? This is becoming pretty frustrating.

edit: nevermind. just realized i had accidentally pinned something


shoulder.blend (70.4 KB)

The problem is with all the tris in this model. Take a look at the attached blend to see how it unwraps with a clean quad mesh.


shoulder_redo.blend (70.3 KB)