I'm a bit late from this bandwagon, but I'd also venture to guess it's the heat waves & sand in the air (seems a bit yellowish to me). Focusing through these is difficult to any AF-system, they cannot detect the presence of heat haze. The other possibility is a bad filter, try it out at home on mid day to see whether it degrades the image or not, I have had it happen to me with telephotos, but not with wide-angles. Your luck might be different.
I haven't figured a way of getting good landscape shots during mid-day, but that's my limitation, there's bound to be someone who can, but I'm not also a full time pro. I have climbed on top of a volcano to shoot about a hundred of photos there at the height of 3 kilometres, not realizing that the metering is different from sea-level due to 30 % more solar radiation and considerably less atmospheric haze - you can imagine the success rate... Should've brought CPL and a ND grad there, but that thought simply didn't cross my mind. But you learn something new every day.
I don't know, it's just a thought of mine, but occasionally it also pays off to exaggerate the natural phenomena. In your case, I don't know how it would have looked like if shot wide open and slightly closer to the camels so that they would be in focus and the background not. But then it wouldn't have been a landscape shot to begin with.
I wouldn't strictly adhere to rules of thirds, it's sorta saying that averagely stuff looks better if placed here, keyword being averagely. When on vacation and I was carrying a 28/1.8, I think I did have to apply a bit more contrast on the scenes to get them more to my liking compared to 85/1.8. But that's just a tad more, and I haven't tried 28/2.8 either.