Judicial activism, as it is understood by its critics, is not the judicial practice of actively overturning laws or precedents, rather than passively upholding them. (Sometimes laws and precedents should indeed be overturned.) Instead, judicial activism refers to the judicial practice of straying from the actual meaning of the law when interpreting and applying it. Judges who engage in judicial activism undermine the American system of government by usurping the role of the legislative branch.
Defenders of judicial activism often tout a "living Constitution" judicial philosophy, which says that the meaning of the Constitution changes and evolves over time and should be interpreted by judges accordingly. But having the Constitution change according to the decisions of judges is the same as having no Constitution at all, and simply being ruled by judges. The very point of a constitution is to have a "fixed" governing document that dictates the requirements and limits of government.
If we wish to change the Constitution, the Constitution itself offers us a means to do so—the amendment process. But the Constitution cannot rightly be changed by a judge simply "deciding" that the meaning is now somehow different. That is judicial tyranny.