My own opinion is that valve springs are subject to conflicting accelerations at high revs. A compressed valve spring that is suddenly relaxed upon valve closure has some harmonic motion, or wobbling/vibration that takes a finite period of time to dissipate. If the spring is violently compressed before this residual vibration has stopped, the acceleration of the spring from compression may collide with the vibration going the opposite direction, creating shear forces that are localized in a very small area. This would accelerate metal fatigue in that small area and cause breakage.
Read the link below about causes of spring failure. Several other possible causes are listed, many of which can be directly extrapolated to valve springs at high rpm.
http://www.lapham-hickey.com/Resources/CausesofSpringFailure10.cfm