Without a direct answer from Malcolm Turnbull, it's difficult to say why, exactly, he decided to do it when he did. However, as Tim Malone has stated (and provided some good reasons) - it's possible to speculate.
In my opinion, there's one main reason as to why it was called now: control of the Senate.
Since the last election, the Coalition Government (LNP) has enjoyed the majority in the House of Representatives. As this page on the Composition of the 44th Parliament shows, the coalition has 90 of the 150 available seats. A comfortable majority which allows them to pass pretty much anything they like in the Lower House.
The Senate, however, is a very different story. As shown in that handy graph, of the 76 Senate seats, the Coalition holds only 33, less than the majority needed to effectively legislate. The breakdown, for those who may not be able to follow the link, is as below:
- Coalition: 33
- Labor: 25
- Greens: 10
- Independents: 4
- Palmer United: 1
- Liberal Democrats: 1
- Family First: 1
- Motoring Enthusiasts: 1
This has left the Senate with a crossbench of 18 members, the highest in Australian political history. And while the Labor Party doesn't have enough numbers to automatically block anything the Liberal Party put forward, the flip side of that is that the Liberals don't have the numbers to push anything through - meaning they have to make deals with members of the crossbench to get any measures supported.
Because Senate terms are usually 6 years, half are usually up for grabs in a normal election, as was planned for this year. The LNP's biggest problem, however, is that of that 18 member cross bench, 11 of those Senators were elected in 2013 (meaning their term would expire in 2019/2020). This ABC page shows the senate results from the last election in 2013.
So if the election were to continue as normal this year (without the Double Dissolution) - of the 38 Senate seats up for grabs, the LNP would have to retain their existing 16 seats and win an additional 6 seats to gain the majority of 39 Senate seats.
Given the rise of the Greens over the last decade in particular, this is far from a certain event. What the government is hoping to do, however, is reduce the number of independent/crossbench senators and, subsequently, their reliance on them.
It is a gamble, of course. But as PM, Malcolm Turnbull has nothing really to lose. The LNP don't currently control the Senate, and it was due to be an election year anyway. This provides the opportunity for the government to start afresh with a 'clean slate' as it were.