Why did the French bomb the Rainbow Warrior?

The Greenpeace flagship, the 'Rainbow Warrior' arrived in New Zealand in July 1985 in preparation for leading a flotilla of boats to Mururoa Atoll in the Pacific Ocean to protest against French nuclear tests on the atoll. Just before midnight on 10 July 1985, underwater charges which had been placed by frogmen on the hull of the Rainbow Warrior, exploded, sinking the vessel while it was in Auckland Harbour, New Zealand. One person was killed in the explosion, Portuguese photographer Fernando Periero. On 22 September 1985, Prime Minister of France Laurent Fabius admitted that the bombing had been carried out by 2 secret agents belonging to the French foreign intelligence agency, DGSC, or Directorate-General for External Security.