Try loading the card and see if it goes back to 16x 2.0.
If it does, it's just an idle power saving measure you've noticed and is nothing to worry about (changing window's power profile to high performance might stop it, if you feel the need to).
...rightful liberty is unobstructed action according to our will within limits drawn around us by the equal rights of others. I do not add 'within the limits of the law,' because law is often but the tyrant's will, and always so when it violates the right of an individual. -- Thomas Jefferson