What's the worst way you've ever been knocked?

@asukojo Hmm, I think I have a clue on how to reproduce this bug. Sit in the train stuck between a teammate and a wall, then have him switch weapons. His loadout needs to include large weapons, such as a Sentinel or Kraber and maybe he needs to have a higher ping.

The instant death happens because of the physics engine. It detects a collision between your hitbox and the environment, but can't push you behind as there's no space. It seems that, in your case, you teammate switched weapons and, as he added the sentinel on his back, his hitbox increased in size and you were suddenly in a a smaller place in the next frame. The fact that you were on a moving platform didn't help either,, because that only amplifies coordinates errors, it's also possible that the server determined he's in a different position and applied a correction, crushing you as a result.