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. You can run LuxMark benchmark and after about 2-3 seconds fans will start spinning.