Why do fans on my graphics card are stopping when it's idle?

It's perfecly normal and it's not a fault of the card.

Most of the latest graphics cards use FAN STOP feature which stops fans to zero when there is no load in order to reduce noise to 0 dB and extend life of the fans. On some cards fans may spin for a while on boot then stop. On others they might only spin when there is load or only one of the fans would spin.