It starts from the idea of being in a confined place such as a grave by mistake. It’s a fear of premature death.

Six Feet Deep

 For as long as I can remember, I have had a deep fear of being buried alive. More specifically, I was worried that the people around me would find me in the form of suspended animation. Resulting in not being able to tell that I was really alive, and they would think I was dead and bury me. 

And then I would come to in a dark coffin with little oxygen and not being able to move. I would terrorize myself with this thought while trying to go to sleep at night. Why would I do that? Well, I have always been fascinated by the macabre, even when it scares the hell out of me.