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.