Twister,
A displacement can also take place when your just very junior on a fleet and another fleet is displacing. Say your on the ATL 777B and the final hammer drops on the DTW 747B in a couple years, at that moment, the 747B can select the 777B (assuming he's senior to you) and displace you, and which you can return the favor to a more junior pilot on another fleet and so on, and so on, which as you can see why displacements can be very expensive for the company and they generally will go to great lengths to mitigate the possibility of displacing pilots, especially when retirements and small growth are available to soak up excess pilots in a given category.