Operating conditions too severe for the design. Elastomers and some seal faces are sensitive to temperature extremes. Excessive pressure can distort seal faces causing them to go out of flat. Excessive pressure can cause elastomer extrusion. High speed can separate the seal faces in rotating seal designs.
High speed can cause excessive heat at the seal faces.
Excessive shaft movement separates faces also. Inner seal blows open if barrier fluid pressure is lost. Inner stationary face is not positively retained to prevent movement if the pressure is lost between the faces. When the outboard seal fails the inboard will fail also due to the pressure drop between the faces. The inner seal has to move into the sealing fluid as the face wears.