The assumption here is that this poor woman panicked and that contributed to her death. Someone who inadvertantly sucks in seawater and goes into laryngeal spasm, or who has an arrhythmia or heart attack, will began thrashing and fighting for air. This is, technically, panic, but it really can be called another word: dying. A person shot in the chest will panic as they die, but panic here is natural and does not contribute to the death. It is a natural consequence of our impending demise.
If you can't get a breath, and have no foseeable way of ever getting one again, you will panic, I guarantee it. One of the hallmark signs of a heart attack is the fear you see in the person's eyes. By it's very nature, impending death in an awake person can induce panic.
In this case, we don't have enough info to solve the chicken-or-the-egg question: did she have some non-lethal event, panic and drown, or a lethal event which will make anyone, no matter how strong and well-trained, panic?
Was the feeling of too much weight really a cardiac event?
When people die of cardiac events immediately, even a post-mortem examination may not reveal the answer, as not enough time has elapsed for pathologic changes to occur.
So, since we may never know, we should give a fellow diver the benefit of the doubt and say she died from something beyond her control, not because she had an irrational panic attack.