The only thing the PML(N) and PPP had in common, besides despising each other, was their mutual hatred of President Pervez Musharraf, which seems to be fleeting. So when these parties announced that they were forming a coalition government, everybody knew it wouldn't last long. Parties driven by cult of personality politics like the PML(N) and PPP don't like to share the limelight.
For now, the PML(N) is only withdrawing from the cabinet, but will continue to support the coalition government. But this is just a pretense. Another excuse will be manufactured to exit the government altogether. This will either mean new elections, or another intervention by the military. Either way, President Pervez Musharraf will be pleased as punch