It is released by normal Mummies when they die, like an automatic Poison Explosion cast on death. The in-game description says that the chemicals that were used to create the Mummies created immense pressure in their bodies, which exploded out when the body was damaged. This appears to be a perfectly logical explanation from a physical and gameworld point of view.
The Poison Cloud is also seen as a trap in the Deserts of Act II and Jungles of Act III. After the trap is heard, a small Poison Cloud is created which rapidly multiplies in the area surrounding it. Sometimes, these clouds can be quite deadly in Hell difficulty.