Answer:
Sugar is interesting stuff. When you cook most foods, the heat is generated by the metal of the pan transferring heat from the flame or burner. But sugar actually creates additional heat as it breaks down, making the temperature rise faster. this means that the sugar can actually be hotter than the parts of the pan away from direct heat. If you stir it, the hot sugar comes in contact with the cooler sides of the pan and crystallization sets in.
In the same way that unmeted chocolate added to melted chocolate will temper the entire mass by crystalizing it, a single sugar crystal from wet sugar that touches the sides of the pan and evaporates, then falls back into the melted sugar will begin the process of crystallization and ruin an entire batch of caramel. Removing it from the heat and adding a little water to liquify it will make it possible to start over.