NidFact: While Norn Queen's death greatly weaken Hive Fleets, it'll also cause other Hive Ships nearby to produce it's own Norn Queen resulting "splinter fleets" creation. This phenomenon is called "Hydra Effect" :
Apocalypse Faction Focus: Tyranid Hive Fleets - Warhammer Community
Warhammer 40K: How The Tyranids Travel The Stars - Bell of Lost Souls