Most likely it is water ingress. Water ingress is when moisture penetrates any product. When this occurs the moisture travels from one side of the substrate (usually brick) through to the other side of the render which absorbs the moisture like a sponge. This causes the bubbling. This often happens on the other side of a wall which backs onto a bathroom or shower, and on the outside of a building where the moisture in the soil makes its way into the outer walls causing bubbling there.