At the end of the Avengers movie, Tony shoves a nuke up the Chitauri's space hole. It goes boom and all of the Chitauri soldiers immediately drop dead.
My thought was that they're robots but in an earlier scene they appear to be some sort of scary lizard creature.
Why did they all suddenly die?
This was apparently explained in the pilot episode of Agents of S.H.I.E.L.D. where they depict a Chitauri neural link that Agent Grant Ward liberates from Vanchat in Paris.
This S.H.I.E.L.D. evidence sheet provides more details (which, despite the numerous misspellings, seems to be an official artifact of the series).
The Chitauri Neural Link is a communication pathway between the Chitauri soldiers and their technology. Neural links are often used to assist, augment, or repair cognitive or sensory-motor functions. They can also be used as a type of mind control and manipulation prostheses. The link connects all Chitauri to each other, to their weapons, and to their vehicles. All information and commands are routed through the Mother Ship which guides the Chitauri forces during their invasion. This makes them a rather formidable fighting force.
....
The Chitauri Mother Ship acts as the main communication hub for the invading Chitauri forces. Not only does it allow their soldiers to communicate but also directs them in battle. The Chitauri are of a hive mind, a singular collective of alien cybor-like creatures working as one to overwelm their enemies.
Fury orders Iron Man to intercept a nuclear bomb and dispose of it. Stark takes it to the portal and blows up the Chitauri Mother Ship, severing the link between the Mother Ship and it's army, thus killing all the Chitauri in New York with a immense feedback discharge.
Writer/Director Joss Whedon spoke to this scene in the director's commentary. In short, the goal was (from filmmaking perspective) to show that the fight was definitively over once Tony had made his hail-Mary sacrifice.
And then we get into the inevitable sort of "Queen bee, uh-huh we killed the head and all the soldiers fall down dead". I'm not proud of that either, Okay? It was necessary to make sure we understood that they didn't have to clean up for the next 17 hours by still fighting. So they could actually have their moment of triumph but it's a device that I'm not fond of (and probably shouldn't have brought up!). (MP3 Link)
Had they not had the "drop down dead" scene, they would have needed additional scenes to explain what happened to the Chitauri soldiers after the spacial fissure closed. There's also a great deal of difference between killing an invading army and killing a soldier who has no hope of escape.
