I don't understand why my Rig interacts with the model a certain way

I have a bone for the Head/Helmet, and when I rotate it, the model comes apart.

Rotating the Headbone, cracks in the face:

When I rotate the Neck Bone, the front of the Helmet stays intact, but the back still stretches the same way like when I rotate the Headbone:

Why does this happen? I think I can fix it, by fixing the model, but I wonder why there is a difference when rotating the Neck and the Headbone.
Thanks! (Oh, and I had more pictures and the .blend file to make my problem clearer but I am not allowed to upload them… o_O)

Edit: I found the problem, it just seems to be uneven weightpainting thats not visible. I used the Normalize tool, and it works as expected.