Hard to really analyze what happened not being there, and not reading every post in the thread, and I agree that the guy behind the OP is ultimately at fault.
That said, it sounds like the OP, and certainly the guy behind the OP could have done a better job of anticipating what was happening ahead.
To the OP, when you're riding in a group, don't just focus on the bike ahead; keep that bike in your peripheral vsion, but your focus up the road. By doing so, you might have seen the situation that lead to the yell developing before the yell, and been better prepared to deal with it.
Anticipating what's about to happen is an important skill to develop in group riding. The guy behind you clearly needs to work on it, and it would also help you.
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You could fall off a cliff and die.
You could get lost and die.
You could hit a tree and die.
OR YOU COULD STAY HOME AND FALL OFF THE COUCH AND DIE.