If the water is too shallow to float the dolphin, its body weight will crush its lungs. That's the most likely explanation. Another feature is stress - dolphins are highly intelligent and they stress very easily if they can't move and swim freely.
One theory for this incident is that sonar from navy ships may have disorintated the dolphins, but the sad truth is, we will never know for sure.
If they beach themselves on shore they slowly suffocate. It's like when you're in the swimming pool you weigh less, but in reverse - when the dolphins beach themselves they can't support their own bodyweight, and the lungs are slowly crushed. I think what happened here is that the ones found dead in the water had been beached but the tide had come and moved them.
Also, in this incident we had two species stranding, which is highly unusual. The post mortems so far haven't revealed any illness in the dead animals.
What a terrible shame - poor creatures. And still rescue people are still trying to save them. I am keeping my fingers crossed for the remainder, that they survive and swim into deeper safer waters. x