TL;DR: You did not cover your viewfinder during your bulb exposure, which resulted in light bleed or light leak into your image.

When the shutter is open, the mirror is rotated up out of the way of the optical path so the sensor can capture the light from the lens. Light from the viewfinder is mostly blocked by the mirror, but there is some light leakage.

The solution is to use the DK-5 viewfinder cover that came with your camera, or use something opaque like gaffer's tape to cover the viewfinder, during long exposures.

Google Image Search for "Nikon D7100 light leak long exposure" shows many similar images with magenta and green bands spanning the long dimension of the image. (I would insert some example images turned up, but none of them are licensed CC BY-SA).

• Discoloration with Nikon D7100 and Hoya NDx400 daytime long exposure (dpreview.com forums)
• Red Line across the picture, long exposure with ND1000 (dpreview.com forums)
• d7100 long exposure, shocked to see the result..
• Serious light leak on the D7100 (nikonites.com fourms)
• long exposure strange colours issue...from viewfinder (D700 user at nikonrumors.com forums)

For the last link in the list above, the user posted an update with test images showing two 30-minute exposures with the body cap on. One of them is without the viewfinder cover, one is with. This is a good test, because it shows only the light leakage pattern for that particular camera.

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The following post at dpreview.com has similar (but much more muted) banding issues on a D610. The image was shot with the viewfinder cover in place, but after investigation found that there was some additional light leakage from around the eyepiece area, resulting in the same streak pattern. This verifies that the red/green banding is coming from the eyepiece.

• Anyone know what this is? Red/Green banding on long exposures

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Here I have reproduced the Nikonrumors.com D700 user's test, using my D800E. Here two 16 minute exposures with the lens cap on, with viewfinder left uncovered (left), and with viewfinder covered (right):

While mine doesn't show the distinct wide magenta band across the frame, you can see similar magenta and green placement at play.