Because the heat output of the chip goes up so quickly, and the cooler cant dissipate the added heat so quickly.
Ways to lower your temps, in order of what you should do.
- Make sure there arent any case airflow issues.
- Make sure that the Cooler is mounted correctly, with an acceptable amount of paste (less is better).
- Lower your overclock or at least the voltage, or if unwilling
- Get a better cooler
What is your idle and ambient temperatures?