It’s usually a combination of edge processing and mixed edge pixels, not a natural visual effect.

Two likely causes from the examples are:

• In-camera sharpening/JPEG processing: strong sharpening can create dark/light halos along contrast edges.
• Partially mixed pixels at the subject boundary: at fine edges, some pixels contain both subject color and green background due to sensor sampling, resizing, or other processing. A keyer that treats those pixels too simply can turn that into visible dark edging.

So the source image may already contain some haloing, but the keying step can make it more obvious if it doesn’t handle semi-transparent or blended edge pixels well.

To reduce it:

• Shoot/process from RAW if possible.
• Turn down or avoid in-camera sharpening.
• Apply sharpening after keying/compositing, not before.
• Start with higher-resolution source images so the transition area is a smaller portion of the final image.
• Use a keying method that handles edge blending / partial transparency better.

If the files are already camera JPEGs, there may be limited ability to remove the halos completely.