Thanks for taking the time to post the picture.
It looks like that bolt failed in fatigue due to bending, originating at the base of a thread, cracking about halfway across the section before finally failing the remaining material in bending overload. As others have said this was likely due to the bolt becoming loose either due to the nut loosening or the clamped material compressing. When the bolt is loose you lose the significant benefit that the eye bolt shoulder provides for bending loads, increasing the bending load on the threaded section. It would be best if you could find an eye bolt with the first inch or more of the shank unthreaded when using a long eye bolt with side load.
It also looks like the turnbuckle was pulling on the top of the eye from the black marks and the orientation of the marks on the fracture face, which is not what you want when side loads are applied. Instead you want the eye rotated to be in line with the side load so the turnbuckle can slide around the eye to the best position. This reduces the bending load on the shank somewhat by shortening the length of the cantilever above the truck bed. This was probably less of a factor In the failure than the loosening was.
