Harper is a formidable foe because his Alberta base ensures that his party has by far the most money to spend. He uses that money on negative, character-based attacks on his opponents, and these attacks have been very successful. As his mentors in the US Republican party have discovered, lies are very effective in undermining your opponents.
2. Last December, Harper pulled such an egregious stunt that he lost the confidence of Canadian parliament. Since he did this a month after an election, an immediate election was not an option and the opposition parties were forced to form a coalition to replace him. Unfortunately, the leader of the official opposition proved himself to be not up to the task of leading the coalition government and had to be replaced. The new leader needed some time to get up to speed. Ever since last December, Harper's government is alive only until Ignatieff is ready to go. Now, Ignatieff is ready to go.
3. To stay in power, a minority government must be propped up by the opposition. This requires that a minority Prime Minister must work with the opposition, or at least with some portion of it. Harper has chosen not to do that, and hence he has teetered during his entire tenure as PM. The instability of the government is all down to Harper. The Liberals and NDP have reached out to him, and in response he has spat at them. Consequently, the decision to topple him is forced on the opposition and their job is merely to pick the most advantageous time. As to when the most advantageous time is, the political experts seem to think it's now.
4. Last fall, Harper broke his own rules and commitment to fixed election terms, and called an election at a time advantageous to himself - another month or two and he wouldn't have been able to keep the deficit figures from the public and the election result would have been quite different. The opposition must take control of the game or we're doomed to a series of weak, unpopular, minority Conservative governments.
5. Harper is still up to his Nixonian scheming, trying to subvert democracy. Years ago he let it be known that his strategy is to beat the Liberals by bankrupting them. He's still at it, with his latest scheme involving trying to force the Liberals to pay retroactive GST fees. There is no reason why the opposition should continue to let him pull these sleazy stunts. Furthermore, the incompetence of the Harper cabinet has led to serious consequences and it is the obligation of the opposition to put an end to it, for the good of all Canadians.
Under Harper's control, the Conservative party has become a gang of thugs, sleazebags and imbeciles. Harper's need to control everything has resulted in a weak and scandal-prone cabinet. Canadians died because of secretive attempts to reduce food safety inspections; the trust of our allies was diminished by a cabinet minister who left NATO document at his mafia-connected girlfriend's house. It just goes on and on, and Harper doesn't seem to be learning or improving. Time to go.
See also: My thoughts before last year’s election