Its a darkframe substraction. You can check what happens without the darkframe substraction for yourself, go to "Raw Parameters" and set the "Dark Frame Substraction" option to "Off" - the camera then wont take the darkframe and present the photo directly to you.
Little explanation: The longer the exposure time, the warmer the sensor and its surrounding parts get - they introduce artifacts known as "amp glow". With very long exposures, this amp glow can become that strong that 1/4 of your image is bright purple, even when you close the lens with your hand. The darkframe takes a picture of the same exposure with a closed shutter, and substracts it from your real photo - that way amp glow and some static hot pixels should get removed.
