The Combustion potion effect is inflicted upon an entity through the Potion of Combustion. It causes an unseen fuse to be lit on the entity, and once the effect wears off, the entity spontaneously combusts, killing itself and and anything in close proximity.
The explosions caused by this effect do, by default, cause damage to the terrain. However, a flag has been set up, where setting the mobGriefing gamerule to false removes the effect causing damage. Example usage of the gamerule: /gamerule mobGriefing false