What causes halo's?

I'm still learning a lot about using dslr's and recently switched from a 40D to a 7D. Now I feel like I need to learn a lot more to get better images that I know this camera is capable of. I was shooting pics of our kids at our neighborhood easter egg hunt this afternoon and got this image...
I used spot focus which was exactly on the egg, ISO at 250 (thought it was on 100, left it at 250 after a shaded tree shot), f2.8, 1/2000. It seems that several of my shots with this camera have had similar halo's around the main focal point...even though this one is not in focus. I used f2.8 on purpose to see if my camera needs micro adjusting. I already took it into Canon once and seemed to get better shots immediately but haven't really put it to the test yet. Lens was 70-200 at 200mm with egg about 15-20 feet away.

Any tips on what you think I should do differently or what I should learn more about to help me with this camera would be appreciated.

HOSTED PHOTO
please log in to view hosted photos in full size.

Only things I can think of are: UV filter? If you have one on try taking it off. Focus seems a bit in front of the egg. Try stopping down just an eensy bit (though with the 70-200 this shouldn't really be an issue I don't think).

Oh, and note that the camera will lock on the closest thing in the focal point (or so I've been led to believe), so maybe it caught some grass or that little flower sticking up?

my 2.8 telephoto has halo's wide open as well but i figured it was a "non L" thing, try stopping down to 3.2 and see what happens

What causes halos?

Depends on your holiness? (har har)

It appears that you focused on something in front of the egg or your camera/lens combination has a severe front focus condition. Can you post a photo of a subject and "halo" that is in focus?

If you have a UV filter on your lens - take it off, throw it on the ground and stomp on it until it is in a million pieces so that it will never ruin another one of your images.

Out of focus image, overexposure, shooting at wider/widest aperture, and the (in)ability of the lens to compensate for them...
it's called some kind of "aberration" ("spherical" perhaps) and it can be found on many lenses especially fast lenses at wide open apertures, lenses otherwise considered excellent.......
It can be eliminated by reducing one or all three of the above variables found in your example....

Thanks for the input...I suppose it was a bad time/place for testing the focus in the bright sunlight and not paying close enough attention to what I was doing.

I'm not a big fan of filters and rarely use them. I kind of figured it was "user error" but the real question was where. None of my other shots today had halo's but they were all shot between f4 and f6.3 and were properly exposed. Whether or nor the lens/camera is front focusing is still up for debate.