That is an interesting observation. They happen after the start, not towards the end. That points to the suddenness of the start in unfamiliar conditions being a major factor.

I recently did a cold water tri mass start and even though I've done tons of them, I could totally see things getting out of control. It wasn't a huge tri, I'm an excellent swimmer, and it was an Xterra, so I figured I'd place myself up towards the front middle because it would be more "casual". The start was insane and even though I finished 3rd or 4th out of the water overall, I spend the first few minutes getting shoved, pushed under, and basically fighting a whole bunch of people around me who all thought they were Phelps or something. The water was mid-50s and I could feel the pressure building up in my neck against the wetsuit as I was having to swim as hard as I could to try to survive the stupidity that was going on around me. I thought to myself "oh shit, this is how it happens. This is how people die during the swim." Luckily, I had warmed up by swimming a few minutes before the start. If I hadn't taken the edge off, I could totally see a different outcome. After the first few minutes of start drama was over and people fell off the pace, I was swimming fine and finished great. I talked with another guy as we came up the ramp to T1 asking WTF was all that b.s. about.

The most dangerous thing in the water is other people. I'm starting on the outside edge from every race here out after that fiasco.