The fluid can flash or vaporize between the faces. Viscous fluids open seal faces as they restrict seal movement. Products that solidify will open and damage seal faces. Crystallizing products restrict seal movement and open the faces. Film building products cause the faces to open.
Hot oil is typical.
The fluid can attack one of the seal components, especially the elastomer. All chemicals have the potential for corroding a seal component. It is just a mater of time. Color contamination problems as the carbon wears. Slurries clog up the sliding seal components and open the faces.