If you are running a stress test or a benchmark such at Furmark that can puch a card to its limits and causses an excess power draw the card will downclock itself to protect itself.  If it does not downclock itself it can damage the card and possibly the power supply by pulling more power than is safe for the power supply to give.

If a card is downlclocking itself under heavy load we recommend testing the card with a program we provide called OC Scanner.  It is located under Support->EVGA Software/Utilities->EVGA OC Scanner.