The campaign pattern of the original Starcraft, and it's Brood War expansion, is mirrored perfectly in the Starcraft 2 pattern of campaigns. In the original, it was Terran, Zerg and Protoss campaigns to begin with, then Protoss, Terran, Zerg in Brood War. Before the epilogue battle, "Into the Void," the expansions of Starcraft 2 fell into the same pattern, with Terran being the first, Zerg the second, then Protoss for the third. In the finale, the Into the Void epilogue, the battles go from Protoss control to Terran and, finally, to Zerg.
If you double-click on the "Hostile Forces Detected" icon in the mission preview, it will change to "Hostile Dancing Detected" and show them dancing.