Because of the energy of the arc load there permanently melts a little bit of the electrodes away. At the moment of igniting some small pieces even drop off. When the lamp ran some time with its highest temperature, this effect decreases because of the reversion of polarity of the square wave voltage. A further wear and tear results from the crystallising glass that is not able to bear up under the very high pressure and the heat. The glass can break loudly. This explains why lamps run longer in the Eco Mode because they produce less warmth and so the glass crystallises slower.