A couple of principles are involved.
Good ol' Bernoulli's law is one. If the velocity gets too high, the pressure goes low, and the liner gets sucked out or at least displaced. THe surface can flutter, and its just a baad thing. A mark of quality in a rubber-lined butterfly is that the liner is mechanically locked to the body.
Second: Any particles in the system get the same additional speed. With the SQUARE of velocity comes Kinetic energy. The chunks hit harder and they do more damage. It is not unlikely that at higher velocities there will be more particles dislodged from upstream to cause damage anyway. Just like when it gets really windy at the beach and it picks up sand. Ouch.
