Honestly, you should have stopped. I know the instinct is to not get involved for fear of being blamed, but the fault almost ALWAYS falls on the person in the back, when it comes to rear-ending. The rules of the road dictate that the follow car follow at a safe enough distance to stop, no matter how sudden, without hitting the car in front. That's where the whole "brake-checking" thing came from for shitty people.

But I wouldn't sweat it. Someone else probably saw it too, and was able to give witness.