That's a real puzzler, 3 in the same race, and not anywhere near the finish. Could be a statistical anomaly, or maybe there was something bad in the water at an early stop?
There are occasionally marathon deaths, and I usually attribute them to people with a condition that probably would've gotten to them soon or later anyway...mowing the lawn, shoveling snow, something strenuous like that. Often the running will delay it, and perhaps in countless cases for a very long time, but once in awhile it strikes.
One danger I've heard about marathoners is that many are so set on finishing that they push too much through a bad time. Top marathoners will usually drop if they're having trouble, because they're rarely in the lead pack if they are. Midpackers maybe should drop when struggling, though of my 13 marathons (or ultras), only 4 of them went really well for me, a few more had difficult times that I got through, and a couple really went poorly. I dropped once, because my feet/knees/hips/butt/back were all hurting, but I didn't feel like I was in any danger.
Another danger I've been reading about since these deaths is undertraining. You really need at least 1, if not 2 or 3 runs of 20 miles or more, and you have to gradually work up to them. Plenty of people make it without those, but that doesn't mean you didn't raise your risk level for the race.
The best advice I've heard is, train for a marathon, but don't run it. You get the health benefits without the stress of running a long, hard race.
This is coming from a guy who's run 2 50Ks in the last month, and is runnimg the NY marathon on Nov 1.