This looks to me like either lateral or longitudinal chromatic aberration.
Lateral chromatic aberration occurs in a lens system when the lens does not have exactly the same magnification for light of all wavelengths. Hence the image in red light is a different size to the image in blue light, and so white highlights have fringing.
Longitudial chromatic aberration occurs in a lens because the different wavelengths of light don't come into focus at precisely the same distance behind the lens. In other words, (say) the green light is in best focus and the red and blue light is a little more out-of-focus.
I think you should be able to distinguish these two cases; if the purple fringing occurs behind, say, the focus point and there is green fringing on the near side of the focus point, then you're looking at longitudinal CA.
[edit: also for the opposite way around, purple in front and green behind, as turns out to be the case here; the point is that the aberration depends on distance behind/in front of the focus point.]
Otherwise, if there is no in-front/behind pattern to the fringing, it's probably lateral CA.
As Hasin suggests this doesn't always happen. Or rather it does always happen at wider apertures, but it's not always visible. It's more visible in areas of strong contrast: sunlight on water, silhouettes, and that kind of thing.
If you are shooting in conditions where you worry that either type of CA may be a problem, just stop down; stopping down reduces the impact of almost all types of optical aberration.
Some (but in 2012 by no means all) digital cameras include image processing code to reduce lateral CA (but not longitudinal CA, since that simply affects the colour of everything in front of or behind the focus point).
There's a good article on this here: http://toothwalker.org/optics/chromatic.html