Feeder size can play a role. If they are to big or sometime consumed to quickly. I am not sure what you ment by the crickets cluster on the mealworms? It is possible for a chameleon to catch more then one bug at a time. If your jackson tried to swallow a couple crickets and a mealworm that could make for a sick stomach.
To much water could do this. I would think normally this is probably not a concern but if you are manually misting and they take it directly in it could happen. Possible if they consumed a lot of cold water or at least this has happened to me.
Death rattles. If your chameleon is showing no other signs of being ill I would probably dismiss this one.