Karen, We have firsthand experience with this. For my brother, it happened because his blood pressure crashed severely at the beginning of dialysis. After leaving the unit by ambulance, the hospital confirmed seizure and we were told what okarol says, that there has to be a predisposition to seizures. We were not aware of such a predisposition in my brother...
As you said, you need dialysis - to remove toxins from your blood and to maintain you at your "dry weight". If you walk in at your dry weight, then no fluid should be being removed. PERIOD.
Karen, start screaming! Be heard, tell your neph, tell the charge nurse that you don't want fluid removed. Even if there is a minimum amount that must be removed, the fluid can be put back (saline) at half hour intervals so that you are never dry enough for your blood pressure to drop. Insist that they do it and get your nephrologist to back you up. Sullidog recently fought this battle and won:
http://ihatedialysis.com/forum/index.php?topic=18356.msg316510#msg316510