Because it is the best chance for Harper to get re-elected right now, that is why he is going to ask the GG to dissolve parliament and call an election. If he gets re-elected, the minority (hope not majority) will probably last another couple of years. Otherwise, if he waits, they may lose popularity, and may lose if the election is called later in the fall or spring next year.
1) The Conservatives are leading in the polls right now (i don't get it, especially due to the listeriosis outbreak, where the Conservative made changes to the meat inspection program, where the meat inspectors no longer inspect, they just review paperwork. They are proposing to allow the companies to monitor everything themselves, and want to change the regulations so companies, not the government is responsible for the day to day inspections. So I guess the fox will be guarding the hen house - sounds like a brilliant idea).
2) There will be parliamentary hearing in the fall on the Bernier scandal, more Mulroney-Schreiber stuff, and investigations/decisions into how the Conservatives violated the election spending limit laws
3) Down south, the Obama "time for a change", doesn't bode well for Harper. It may have an impact on the psyche of the public here, especially if Obama and the Democrats win.
4) The economy is falling. Conservatives want to call it now before the real bad economic numbers come in a few months from now.
The Conservatives passed an fixed election law, so the election is supposed to be fall 2009. When they passed this, they said this will prevent governments from arbitarily calling elections when it suits them politically. Now this is exactly what they are doing -hyprocrites.