AI Super Jump
From Halopedia, the Halo wiki
Halo 3: ODST
In Reach, the glitch is usually triggered when a Grunt fires their weapon or is shot while their model overlaps with a cargo box. Doing so will send them shooting into the sky, only to fall back down again. Fall damage may not apply, but collision damage does, so the Grunt may survive the landing, but not survive if they collide into a wall. If the shot that sent them into the air was a killing shot, the flight will delay the Grunt's death until they hit the ground.
The glitch can occur in any level, but most commonly does on the Firefight map Courtyard, due to the plethora of cargo boxes that are present.