From Halopedia, the Halo wiki
Air Swimming is a phenomenon that happens in Halo: Combat Evolved
This phenomenon occurs when an enemy is killed by a grenade explosion and their body gets stuck between two terrain objects. The Halo game engine necessitates that every in-game object have its own 'box' of dedicated space. This 'box' is usually what the player runs into, instead of the object itself. When a lifeless body is too large to fit in between the boxes of two objects, it ends up hovering above them, looking as though it were trying to swim through the air. The event occurs most commonly with Jackals.
In Halo: Combat Evolved, when bodies are propelled by explosions, they will sometimes appear to be "frozen" if they hit an object in mid-flight; this can also make them appear to be swimming. On a related note, when a Jackal or a Grunt is killed by an explosion, its corpse may appear "wedged" within a metallic structure, floating above the floor.