Malcolm Turnbull has called a double dissolution election for the following reasons:
(1) To get a personal mandate from the Australian people. Turnbull currently occupies the prime ministership because he overthrew Tony Abbott through an internal party spill. Notwithstanding that there were a lot of excellent reasons for Abbott to be deposed, there is a lingering question of Turnbull’s legitimacy to hold the office of PM. Winning an election will give him a personal mandate from the electorate.
(2) As mentioned by some other contributors, the minority parties have been a thorn in the side of the government. One goal of the this election is to get rid of the minority parties from the senate once and for all.
(3) Turnbull’s personal popularity and more particularly the popularity of the LNP government has been declining since the start of 2016 and it is continuing to decline to the point where it’s now too close to call.
Barring something really unforeseen it’s pretty certain the government is going to lose seats in the election. The sooner Turnbull called the election the fewer seats the government is likely to lose.
It’s mainly to get rid of the smaller party senators, and a chance to get re-elected before the possibility of entering Australia’s first economic recession in over 20 years.
If he doesn’t call a double dissolution now, he will have to call a general election in 6 months time.
By then, only half of the seats in the senate are up for contest. They exclude the seats won by the minor parties (the Palmer’s United Party, the Motoring Enthusiast’s Party, and the Australian Liberal Democratic Party) at the last election, which have been troublesome for his government. With the recent senate reforms, it is less likely that these smaller party senators would be re-elected.
Also, the economy has not yet fallen into recession, which may be a realistic possibility in six months time. A government is far less likely to be re-elected after it led the country into an economic recession, so it is more preferable for it to pull the trigger now for a double-dissolution election.