Why did my salmon turn brown when I steamed it?
Most sources recommend anywhere between 5-10 minutes to steam a piece of salmon.
Bonappetit.com says:

Although oily fish can be easier to execute when steaming compared to white fish because of the fat content, you have to avoid overcooking it, since the bloodline between the flesh and the skin can let off a metallic flavor.

The most likely answer would seem to be that, in steaming for 20 minutes, you massively overcooked the fish.