If it is raining or storming impulse command blocks that are set to summon LigthningBolts summon a lightning bolt regardless of redstone settings the first time they recieve a block update. Repeating command blocks also summon and continue to summon ligthning bolts if the same conditions are met. Changing to clear weather stops them but they resume when the weather is set to rain or thunder again. Changing the weather also triggers this but does not prevent this to happen to the same command block again like providing a block update does.