Why does it take longer to my DSLR to read out a long exposure image from the CCD?

I have a Nikon D3200. I noticed that shots I made with exposure times in seconds also take longer to write to the SD card *after the exposure is finished*. Images of dark scenes taken with a maximum exposure of 30 seconds take almost one more 30 extra seconds to save before the camera is ready to take another picture. I don't understand it - shouldn't the CCD be in about the same state after the mirror jumps back regardless of how long it had been opened before? Is the camera post-processing anything?

The camera is taking a second "shot" as a dark frame reference. When you take a long exposure, the sensor heats up and this can create digital "noise" in the photo just like at very high ISO.So, when a long exposure is taken, the camera creates another exposure at the same time and maps the digital noise. It then uses this as a dark frame subtraction reference to remove the noise from your "real" photo. It is a good system to help keep noise out of long exposure photos. It can, however, be turned off if you want. Look in your owners manual to learn how to disable this feature.

Your camera is taking a second "shot" of the data from the sensor for dark-frame subtraction. From Wikipedia:

In digital photography, dark-frame subtraction is a way to minimize image noise for pictures taken with long exposure times. It takes advantage of the fact that a component of image noise, known as fixed-pattern noise, is the same from shot to shot: noise from the sensor, dead or hot pixels. It works by taking a picture with the shutter closed.

A dark frame is an image captured with the sensor in the dark, essentially just an image of noise in an image sensor. A dark frame, or an average of several dark frames, can then be subtracted from subsequent images to correct for fixed-pattern noise such as that caused by dark current. Dark-frame subtraction has been done for some time in scientific imaging; many newer consumer digital cameras offer it as an option, or may do it automatically for exposures beyond a certain time.

Actually your camera does not have a CCD sensor. DSLR cameras have been using CMOS sensors for a long time

As mentioned, with the long exposure NR turned on, the camera is recording two images to one file. Part of the magic of reducing noise using a digital camera