The dough is too soft. It might have sat out too long on the counter before you baked the cookies, or you might have let the butter get too soft before creaming it with the sugar. Chill the dough for a few hours and then bake the cookies.
You measured the flour wrong. If you are not weighing the flour (and unfortunately the recipe on the back of the package does not give you a weight), it is very easy to measure the wrong amount. If you did not add enough flour to your dough, the dough could spread.
You swapped extra-large eggs for the large eggs called for in the recipe. The extra liquid in the large eggs would make the dough looser and possibly spread more.
The cookie sheet was too hot. If you only have one cookie sheet to bake the cookies on, let it cool completely before loading it up with more cookie dough.
You greased the cookie sheets. There is enough fat in the recipe that the sheets do not need to be greased. Greasing the sheets will make the cookies spread.
You added more sugar than the recipe called for. Sugar is hydroscopic, meaning it absorbs liquid. Once it is baked the sugar releases that liquid so if there was too much sugar in the recipe, that could make the cookies spread.