What is causing the high degree of chromatic aberration here?

[Quicklabs]

The first shot is a 150% crop of a shot taken with the Sigma 30 1.4. (First pic--I think I probably cropped this a little bit before.) I am using a CP filter. There is an astonishing amount of CA in this picture. I've noticed several other pictures of this type (Tree branches against sky_ taken on the same day and about the same time, same filter and lens, with similar problems. The CA tends to show up in the same part of the frame. At other times (without the sky background), I don't see any CA. (Pic 2 shown below, 150 % crop, same part of the frame) Nor do I see it in situations other than high contrast tree/sky? Where CA appears, it is at the far upper part of the frame.
Is this a problem with my lens? Or a filter problem? Using the filter improperly? Or just the high contrast of the dark tree against the sky? Or processing? Or something I haven't even dreamed of yet?
Thanks for looking.

 

[ukcatfan]

I believe the CP could be introducing it. Try some similar test shots with and without it.

 

[GrillMouster]

Chromatic abberation can sometimes be caused by the glass. Sometimes purple fringing in high contrast areas is due to the sensor having different sensitivity levels in the red, blue, and green channels, or the microlenses for the pixels on the sensor incorrectly focus on red and blue. Tree branches against a bright sky, especially around the edge of the frame, is a common place to find the problem. There's not a whole lot you can do about it other than fix it in post production.

 

[Quicklabs]

x

Chromatic abberation can sometimes be caused by the glass. Sometimes purple fringing in high contrast areas is due to the sensor having different sensitivity levels in the red, blue, and green channels, or the microlenses for the pixels on the sensor incorrectly focus on red and blue. Tree branches against a bright sky, especially around the edge of the frame, is a common place to find the problem. There's not a whole lot you can do about it other than fix it in post production.

 

[Groucho]

Wide-angle lenses are fairly prone to purple fringing... just look closely at the edges of almost any fisheye photo! 30mm is still fairly wide, it's probably a little more prone to such things than, say, a 50mm lens.

That particular lens is also reportly one where the edges tend to fall apart a little, Sigma seems to have put the focus (no pun intended) on getting maximum sharpness in the center at the expense of the edges.