In the words of John Cleese, that is an ex-engine. There's about to be quite a lot of fire; the Combine inside start frantically trying to get themselves clear before it becomes a lot of boom, too.

One almost pities the Combine at this point. They seem to have just enough humanity left to be freaking out. There's not a whole lot of them left in the area after Annabelle starts throwing, either. The rebels are pressing forward, and overhead something that gives the impression of being a giant, flying crab darts through the air, passing over the scene on the way to somewhere else. "Dropship!" Dora calls. "Bet you that'll be coming back with some serious nasties inside!"