The temperature of the product has changed. A discharge valve is being throttled A common practice in an attempt to stop cavitation. The impeller has been trimmed. The discharge of the pump is being restricted A foreign object was left in the pipe the last time the system was opened, or maybe the swing check valve seat has come loose etc. There is a buildup of material on the wall of the piping and fittings.
Hard water leaving a layer of calcium on everything is a good example.
Many other fluids can coat out on the pipe walls also. A truck ran over it. A liner has come loose and is restricting the passage. A frozen water jacket has collapsed the pipe. A restricting fitting has been substituted or added to the system.