I went to the Oregon Coast the other day, a beauty, and from Ecola Head decided I would try and emulate the style of the great but sadly recently deceased Chip Hooper with a long exposure of the ocean. I used my Fuji X T-1 and the 50-140 lens at f/5.6 with manual exposure of 6 minutes through a combination of a 3.0 and 2.1 neutral density filters. The exposure was pretty close but when I got home, I noticed horizontal banding in the image. I have used ND filters with some frequency to slo moving water and have even used this particular combination when it's quite bright without difficulty but usually for shorter exposures. I did a test in my home with a similar set, both filters, 6 minute exposure up but got only noise and color shift, as expected, no banding.
I've attached a cropped section of the file so that the banding might be more easily seen.
Any ideas as to why this happened?