That's what happens to every DSLRs. You're using an optical viewfinder, and it's called optical for a good reason.
Light will pass through the lens, hit a mirror angled at 45 degrees, reflect upwards and hit a pentaprism. It'll reflect around in a slightly complicated manner in the pentaprism and exit through your viewfinder. No electronics involved. That's why we say EVF and OVF. Electronic and optical viewfinder. EVF is like a Live View function. DSLR stands for Digital Single Lens Reflex. The "reflex" refers to the mirror flipping.
Behind the mirror is the shutter — 2 sheets of thin material that covers the sensor. When the shutter is released, the mirror flips up to make way for the light. Then, the shutter opens up to expose the sensor, which takes in the light and converts the values of the light to digital data. The data is then processed (eg. sharpening, noise reduction, White balance etc.) and transferred to your memory card as JPEG files.
So surely, nothing's wrong with the mirror flipping up. Instead, something is wrong if it doesn't.
But of course, there's much more factors.
-tripod not steady
-surface where you place your tripod vibrates easily
-VR mode turned on when it shouldn't be
-mirror flap
-subject changed distance after you locked your focus
-motion blur
-manual focus not focused properly
I'll just explain about surface, VR and mirror flap.
Some surfaces aren't so nice. Even though your sensor sometimes only gets exposed to light for like 1/300 seconds, it's longer than you think. It takes some practice to know the right shutters, but even then no one can really tell how much shutter speed is the minimum to freeze a certain subject. Eg. you can't say at first glance, with only your eyes and brain, that the aeroplane needs minimum 1/300 shutter to freeze. That's superhuman
How VR works is that in the lens, the glass elements will move around. Let's say you move your camera to the left. In order to counter the leftward motion, your lens will detect the leftward movement and move the glass elements to the right. But if you don't need VR and your VR is on, the lens will try to search for movement and the glass elements will move randomly even though there's no vibration. So when do you need VR? Look at your focal length. Let's take focal length to be F. 1/F is the minimum shutter speed you need to stop the vibration. So if your focal length is 100mm, you'll need 1/100s to stop the vibration. Similarly, 200mm will need 1/200s. 160mm will need 1/160s and so on.
When the mirror flips up, because it's built to flip so fast, it'll more or less slam onto the top. When the mirror slams, it'll cause some slight vibrations in the camera. Some cameras like the D7000 has a Mirror Up mode. You release the shutter button once, the mirror flips up. The shutter only opens when you release the shutter button the second time.
Hope I didn't make any careless mistakes
