In the past, there have been incidents of violence by the employers against striking union members. As a result Union leadership has sought special protection by the government against businesses that would commit such acts of violence. Libertarians oppose any special legislation for or against unions, so therefore libertarians oppose this special legislation as well, so therefore libertarians support when businessmen inflict violence against union members.
There are problems with that argument. The first of them is that union members have been known to use violence against scabs. While that is not an argument for or against special legislation, when an argument used in favor of a proposition can also be used against it that is a pretty good sign that the argument is not very sound.
The major flaw is that libertarians are against the violent crime of battery. If violence is used by employers against union members, that means violence is being used against individuals. Any libertarian that recognizes a purpose for government would insist that government use its power to defend the rights of individuals, especially the right to life and the sanctity of the body of the individual.
In such a minarchist system, a businessman who hires people to assault union workers, and any thugs who are employed by that businessman for that purpose, are all criminals. The government should therefore enforce the laws that already exist, and laws already exist to protect striking union members from being assaulted by the hired thugs.
If the laws are not being enforced in the first place, there seems a strange futility in calling for more laws to do what existing laws already cover. That is why it also seems strange that libertarians, who do not believe in assaulting union workers when they go on strike, are considered to be anti-union.