Me messing about.
I think it's from the way I did the sharpening. Rather than just the USM I thought I'd have a play with using a high pass filter on a dup layer with the hard light setting.
I just wanted to see what it did. Seems to work quite well for the grass and the tree on the left but it does give a halo around a dark edge. Quite easy to remove and I did take it out but after I uploaded the jpeg. It's certainly not the best way to add crispness to this shot but quite an interesting alternative if you are getting too much noise across sections of constant or slightly graded colour with USM.