Darth Sidious's rise to power was meant to be smooth and efficient.  Grievous, being the stubborn cyborg that he is, refused to roll over and die.  Now the Empire is chasing him around the galaxy, Grievous is starting insurrections, and Vader is busy putting out the fire. Canon as you know it is completely, utterly, screwed up. Takes place in the Expanded Universe. (Legends Canon)