I know from my own personal reflex to hold my breath in a sudden drop on a rollercoaster which is similar to the freefall a plane could potentially be a cause. I found that on a rollercoaster with several long drops, my chest was starting to hurt and my heart rate was going crazy. Only once I started breathing (screaming lol) on drops did things calm down a bit. If the passenger had a heart condition and did something similar, then that in itself could have caused stress on the heart that could have led to the heart attack.

And it wouldn't even have to be this since anyone prone to a heart attack could have one triggered by any type of stress. Or even not--as in the case of my uncle who several years ago coughed twice while putting on his shoes and then fell over and was gone in 30 seconds with no prior history. It is always sad to hear the passing of a life, no matter how it happens, and is a reminder to take a second in the day to be grateful you're still here.