The scheduling software (Qube) is responsible for retrying failed jobs. A frame render can fail for many reasons (e.g. Incorrectly packaged assets, missing text

The scheduling software (Qube) is responsible for retrying failed jobs. A frame render can fail for many reasons (e.g. Incorrectly packaged assets, missing textures, out of memory, bad lighting info, missing plugins, random SIGHUPs - rarer) - so if something screws up then Qube will retry N times if it can find available hardware. From the artists point of view, they don't render a frame once, they continuously render and progressively increase the level of detail until they're happy with the scene .. this means the render farm is almost always busy.