One common element is the unexpected cause. For example, like eating something strange or being cursed by magic. Another is the feeling of helplessness as the belly inflates. The character can't stop it and often has no idea how to reverse it. Also, the physical discomfort and pain that comes with the inflation, like the skin stretching and internal organs being pushed around.
Well, often there's an initial action that sets off the inflation, such as drinking a mysterious potion or being bitten by a strange creature. Then there's the horror of seeing one's body change in such an abnormal way. The inflated belly also usually causes problems with movement, making the character feel trapped. And the fear of the unknown, not knowing if the inflation will stop or if it will lead to death. It's all about the sense of losing control over one's own body.