A film can build on the hot seal surface restricting movement. This can occur at the lapped faces or in-between sliding components that must be free to flex and move. The product can crystallize with a change in temperature. Vaporization at the seal faces can freeze the moisture outboard of the seal or any oil or lubricant that might have been put on the lapped faces. Hot water experiences this failure.
Critical dimensions change that can affect seal performance.
Pipe strain will increase with temperature changes. The wear ring clearance will change. The O-ring squeeze will change. The interference fit between the face outside diameter and its metal holder can loosen. Corrosion always increases with an increase in product or face temperature.