Why caramel crystallize after adding butter?
Acids, such as cream of tartar, can interfere with the formation of crystals by inverting sugars. Without it, you need to be more careful/lucky... 

To quote Cooks Illustrated [It's for simple syrup but point still holds]:


  Simple syrup crystallizes when enough of the sugar molecules stick to one another that they become insoluble in the water... To find a more effective solution, we had to understand why these additives helped. The cream of tartar and lemon juice are both acids that are able to break down sugar molecules into glucose and fructose in a process called inversion. So not only were fewer sugar molecules available to cluster together in our doctored syrups but the newly present glucose and fructose were physically blocking the remaining sugar molecules from one another.