hi there you didn't list the situations in your question so i will give you the most important situation:
When the protests by the union threatens National security
Explanation:
protests by Union workers is one of the mediums by which workers get to express their unsatisfied working conditions which is most likely to be ( low pay, poor welfare package or poor working conditions ) at work. sometimes union protests about the victimization of one of there members or a group of there members.
Government will have to get involved in union protests when the protests seem to endanger the lives and properties of citizens i.e NATIONAL SECURITY.
I believe the answer you are looking for is: "a violent or destructive protest." The government would be most likely to become involved in a union protest that is violent or destructive.
