I think of segfaults as being caused by accessing an out-of-bounds index in an array, personally. Yes, you can do the same thing by dereferencing NULL, but in my (university only) experience, overrunning an array came up much more often.
In C it is quite common for functions to indicate failure by returning NULL. Not checking for these errors is another very common reason for segfaults.
In C, accessing an out of bounds array index does not necessarily cause a segmentation fault. It's called buffer overrun and regularly used in malicious exploits / attacks.
What does occur many times is you mess up your stack, or mess with other memory in your heap, causing segmentation faults for various other reasons.
