I've seen two things cause that effect. As mentioned above, if the lines are visible on the bare negative, it's probably shutter curtain timing. Especially on higher speeds, the second shutter curtain is catching the first curtain.
The second cause I've seen is in scanning. With a Nikon Coolscan 9000 I get that effect on negatives, especially negatives that have a smooth gradation like sky, when doing a normal scan (the quicker option). I'm sure I have the terminology wrong, as I am not at my scanning computer right now, but it has something to do with the three different scan heads used in the scanner for normal scanning. By using the single pass, or single head, or some term like that scan, it takes much longer to scan the negative, but those lines will disappear.
Hope that's a little clearer than mud.