The Horse of Copinsay

The only shot I took today as both my cameras failed. I went out to Deerness and walked from Lighthouse Corner out to Mull Head. When I tried to use my Nikon D5100 it went peculiar. The shot was massively under-exposed. Then, when I looked through the viewfinder and pressed the shutter release lightly, the image did a little rock back and forth!  I changed the battery but to no effect. Then the battery on the Fuji failed and there was no power in the spare. I did get this shot first, but I am posting more so some expert can suggest what went wrong with the Nikon. I will be sending it off, but don't really want to lose it for weeks.