It's not normal for your back to constantly crack although it is normal for your back to occasionally crack and is often harmless.
If your back is constantly cracking or if it's also accompanied by pain it can indicate an underlying issue that should be checked out by a doctor.
Constant back cracking can indicate arthritis or even a spinal misalignment.
In most cases though the back cracking is a result of the synovial fluid that becomes gaseous when you release the pressure on your back by twisting.
The reason why your lower back pops when you twist is because of the gas bubbles in the fluid that lubricates your joints being released which makes the popping sound.
Another cause of the lower back popping when you twist is because of tendons or ligaments moving out of place which is called crepitus.
Most commonly though it's the gas bubbles in the fluid that lubricates the joints being released when you twist.
Just like when you crack your back or stretch your back and release the gas bubbles.
Stretching your back can put pressure on the fluid that is in your joints and cause gas bubbles to form.
And when you crack your back, the gas is then released and makes and causes the popping sound.
This popping sound is similar to what happens when you crack your knuckles.
When you crack your knuckles you release the gas bubbles and hear the popping or cracking sound.
Also when you twist your back, the ligaments and tendons may also move out of place and cause the popping or cracking sound.
Another cause is vertebrae rubbing which is when the vertebrae in the spine rubs against each other.
Most cases of back popping are normal and harmless but if you experience any persistent pain, limited mobility or changes in the nature of the popping you should seek medical attention.