Free trade can lead to a "race to the bottom" in which industry moves to the country with the lowest taxation, lowest environmental standards, worst treatment of employees, worst data protection, etc. and so other countries are encouraged to similarly drop their standards and everything gets worse for almost everyone, arguably, at least in some cases (probably no one would claim that free trade is a bad thing in all cases).

In the case of environmental standards free trade between countries that do not have the same rules and standards makes it harder to prevent a "tragedy of the commons": I think that's another way of looking at it.

Probably an economist could give a better answer.