Because the enviromental FUD became quite strong recently, ever since journalists started making headlines like "Bitcoin uses more energy than country X", public perception has started shifting to negative. When I saw people on reddit mention Bitcoin in unrelated conversations, the most common sentiment was "what is Bitcoin, what is it useful for?", today it's "Bitcoin is really bad for environment". So some miners want to publicly announce that they are swithing to green to combat this FUD, hopefully increasing the price of BTC.
This is true, the media is over exaggerating things here. Considering that there are many other human activities that are only wasting energy or much more intensive energy users. As bitcoin is relatively new to their grasp, they are finding loopholes on how to find faults on this new technology. This is why miners are now aggressive also in announcing their use of renewable energy sources, for them to stop the FUD surrounding the use of too much energy.