I doubt this has anything to do with something physical, like a filter. Because of the way Bayer camera sensors work they are sometimes prone to creating detail artificially where there was none. I am struggling to find a link to the article I read months ago, but I believe the thrust of it went something like this: the pixels in your raw file do not each map to a single physical pixel on the sensor surface. Instead, they are dynamically generated from information gathered by an array of adjacent "pixels" sensitive individually to Green, Blue and Red light. The camera's software has to make educated guesses as to how to use this information to create a single pixel, and sometimes -- especially in areas with lots of busy detail, like a field of grass -- it can goof and create lines and other patterns that simply weren't there.
This is what it looks like to me. However, as to why it was triggered, I'm not sure I can hazard a guess. You don't seem to have any fine detail in these images that would confuse the camera's software, especially in the first image where we would expect smooth, continuous tones. I do notice, however, that the pattern fades towards the right edge of the first image. Since both examples you gave us are clearly crops from a larger image, I would suggest going back to the uncropped originals and seeing if the area of the frame the the first image corresponds to shows a similar fading of the pattern in the second image. This would give you a hint as to whether it's an issue that varies across the sensor surface in a consistent fashion (in which case I'd think something was wrong with the sensor itself). If you don't see the same fading in the second image in that part of the frame, something else will be confusing the software, I would think.
[edit] Found an article that discusses this somewhat:
http://www.cambridgeincolour.com/tutorials/camera-sensors.htm
Scroll down to the 'Demosaicing Artifacts' section. The second row of (smaller) images, shows a pattern VERY similar to yours, second from the right. Now what I don't understand is what would be triggering your camera's sensor/sensor software into thinking you had fine repeating detail in either of these images.