Things ride A LOT on how well ur neck is aligned and how its doing. Its very easy to get ur lower back out of wack if ur neck is out of wack.
Although it could be coincidental I didnt have any lower back problems until 3 months after my own ACDF.
It could be what kalvin suggested as cervical myelopathy can cause weakness and poor gait problems in the legs. Or the fall could of caused a misalignment of ur lower back causing problems. I has severe spondylolisthesis which caused severe bilateral foraminal stenosis which caused significant bilateral root nerve compression at l5/s1 which resulted in the impairment of the function in my legs. Real weak, abnormal gait...it wasn't fun. Wouldn't hurt to get things checked, take care:)
Are you sure you didn't injury your low back in the same fall? The cervical spine can cause problems in the legs due to spinal cord compression and canal stenosis, this disorder is called Cervical Myelopathy. This condition is something that generally progresses over a considerable amount of time and isn't something that would be typically caused by an accident, but I guess its possible.
Did you have a post-op MRI? If you did, get the films and go see another surgeon.