As almost always, we have almost no information at all about this accident, other than that one diver died at depth and the other at the hospital. Medical issues, oxygen toxicity, hypoxic . . . there are lots of things we could posit as causes of the drowned diver's problems. The diver who surfaced either did markedly inadequate decompression or embolized, it sounds like; he may have been trying to save his partner, or he may have panicked and bolted, or lost buoyancy control for some reason. Most likely, we will never know what actually happened. It's not clear how much, if any information was obtainable from the diver who made it out, nor is there any significant likelihood that such information would ever be shared in a public forum. This is why we learn nothing and nothing changes.