In all shots of the animatronic Leaellynasaura, they have "excess skin" of sorts around their mouths, which juts out in an odd way when their mouths are closed. This is probably due to the material used to create the animatronics not being stretchy enough - it looks fine when the creatures open their mouths wide.
As the dwarf Allosaur kills its prey, it shakes its head violently from side to side. When it is done, it opens its mouth to drop the dead animal to the ground. Its jaws open, its head tilts to the side, but its neck remains still, so part of its head phases into the neck, creating a painful-looking effect.