I agree with CapeJohn. Pacelining has rules, which are designed to minimize the risks; and if ya don't know them, you should either learn them or don't paceline. The leader has to call out all obstacles, and the people in the paceline must pass the word to the back freely. "Hole," "stick," "gravel," "car up," etc. A paceline requires communicating everything, and hand signals aren't good enough. The rider hitting the brakes should have immediately hollered, "braking," immediately followed by everyone to the rear hollering it too. That might not have helped you, but it might have given the guy behind you a half second extra reaction time.