That "arm" was actually a foot, and the fossil bits just kept coming. The site is now the largest find ever for this particular species, which was a large, feathered cousin to the more familiar Velociraptor. It seems that these unfortunate raptors were trapped in quicksand -- sand so heavy with water that it loses much of the friction between its grains. Quicksand isn't actually the deathtrap for humans that cinema would have us believe, but for a frightened animal who couldn't gain purchase, it might have meant suffocation or slow starvation -- or simply getting stuck until a bigger predator arrived to finish the job.
Brian Switek for National Geographic reports that a plant-eating dinosaur was found at the site, too, which could mean that the raptors all died at the same time while hunting the trapped creature. That would be exciting, because despite their depiction as pack hunters in the "Jurassic Park"films, we don't have much evidence about whether dinosaurs like these came in droves or hunted solo.
If the researchers can show that the raptors grew tangled up together as they struggled to get free, or find evidence that the same weather patterns affected their bones when they died, it would add weight to the notion that raptors liked to rumble in gangs.
You can see clips of the excavation in progress over at National Geographic.