The point of a coalition is only this: to assume power, at someone else's expense...in this case at the expense of the Conservatives. That's what coalitions are about, assuming power. The Bloc has many different political concerns in mind, not just that of separation, so there is no reason why other parties cannot form temporary alliances of convenience with the Bloc. It's just normal politics to do that.

In this case Harper brought in a budget that was completely unacceptable to the Bloc and the Liberals and the NDP. Thus he gave them all a very good reason to form an alliance, and he is the author of his own misfortunes as a result of that.

The Liberals are also the author of their own misfortunes, of course. It seems to usually be the way in politics, doesn't it? Give them some rope and they are all fairly adept at hanging themselves most of the time.