Hot GPU

I have an iMac (Retina 5K, 27-inch, late 2015), processor 4 GHz Quad-Core Intel Core i7, memory 16 GB, GPU AMD Radeon R9 M395X, 4 GB.

When I wake up my computer from sleep, the fans start working and Macs Fan Control shows me that the GPU has a temperature of 85° C (185° F).

Why is my GPU getting so hot while doing nothing, the screen is black. And why aren't the fans running when it gets so hot.