LINK180 elements do not have any rotational DOFs. It is a ‘truss’ element. It does not have any bending stiffness. If you only constrain one node, it can freely rotate/pivot about that point.
Some ideas to consider:
1. Apply sufficient restraints to prevent rigid body rotation.
2. When meshing lines with link elements only use 1 element per edge. Without bending stiffness, multiple links in series may have stability issues.
3. If a series of link elements are required, you may need to apply an initial tension to provide stability.