Tractoring over the same ground

So, I have been successfully tractoring some New Zealand Whites all summer and they have adapted to this diet very well. But I'm almost out of ground to move them over. Last year I had a couple rabbits that I tried tractoring over ground that they had been on over a month before. Both rabbits developed wry neck and died within 24 hours. It was suggested that this happened because I ran them over ground they had been on earlier that year, and they you need to wait a full year before using the same ground a second time. Does anyone have any experience with this? How long does ground need to sit before running rabbits over it a second time? Is it a winter freeze needing to kill of some nasties or something kind of a situation?

There are multiple causes of head tilt, and these affect the inner ear, nerves, brain, or a combination of sites.

Head Tilt (Wry Neck, Torticollis)
http://www.peteducation.com/article.cfm?c=18+1803&aid=2371

Did you see drooping lip, drooling, sunken eye, loss of a blink reflex, and the third eyelid covering a portion of the eye?

Did you do examine the body after death as in performing a necropsy?
If so did you see any puss or drainage in the ears?

There are things out in the environment like:
a) Raccoon roundworm, Baylisascaris procyonis, can infect the brain of many mammals including dogs, rabbits, and humans.
b) E. cuniculi a protozoan that commonly parasitizes rabbits.
c) Pasteurella tularensis, is capable of infecting a number of wild birds and mammals (including man), but more commonly occurs in rodents and rabbits.

The above can be brought into an environment by wildlife. Rotation of grazing would have had no impact on a raccoon taking a dump on your property.
The same would go if you have a population of wild rabbits sharing the grazing areas. It is really important to have a proper necropsy preformed on dead animals.
Knowing what killed them goes a long ways to identifying preventative measure to avoid future deaths.