It got triggered by a bill about re-instating the Australian Building and Construction Commission, which was actually about persecuting unions or holding them to account depending on your side of politics. However that was just the trigger, there have been lots of bills which haven't passed in this parliament because the senate was held by minority parties and independents. (One of the minority parties itself splintering into independents). Ultimately the trigger was pulled because with a new Prime Minister at the helm, the liberal party think they can win the election and gain control of both houses.