You know, I think I've heard "cast of crabs" before. But most terms of venery (e.g., "murder of crows" or "parliament of owls") probably aren't "real" (i.e., they probably weren't ever in common use), so I feel like it's probably more accurate to say that there isn't a "proper" word for a group of crabs.
Dark Sun is a pretty big outlier for things like that. It's also unusual in using the Animal type for a lot of fantastical creatures. Most monsters of the Animal type are actual animals that exist in the real world, or are at least closely based on real-world animals (e.g., the Eberron dinosaurs). Dark Sun throws that tradition out completely, so it's not surprising that it would also buck typing trends in other ways.
But, regardless of whether or not you have a problem with it, it is pretty unusual, and it's one of several things about this monster that are just slightly odd. So, it just feels like an oddball monster to me.