Safety devices and contactors are designed to operate with leading voltage, not leading current, on an AC system. When a safety device or contactor opens it is designed to break the voltage which then stops the current flow.
If the contactor opens with leading current, the voltage is still present and arcing occurs. The arcing causes damage to the contact surfaces. The amount of arcing depends upon the voltage present when the conacts are separated. As the phase angle (displacement power factor) becomes more and more leading the instantaneous voltage level increases toward the peak of the sinewave of the voltage and more damage will occur.
The problem is the pitting (chunks of metal are pulled from the metal surface) upon each activation due to arcing. As the amount of pitting increases the actual surafce area left is less and less on the contacts. At some point the contact surfaces are eroded sufficiently to either they weld shut upon the next occurence or prevent closure because not enough surface is present.
Hope this helps.