I had this exact same question when I first bought this lens. Check out this link, it explains everything http://toothwalker.org/optics/bokeh.html
Basically the aperture for the 85 1.2 (aka the "Canon ball") is so freaking big that the light that it lets in gets partially blocked by the mirror box. I think the effect is worse on a full frame cameras and stepping down decreases it as well. You will see this effect mostly when shooting wide open. It is not, as others have stated, due to the light being blocked by something before it enters the lens (i.e. part of the tree) but instead the light is being blocked after entering the lens itself.