Not all nylon ferrules are created equal. Some or just nylon and some have brass (I think it's brass) molded into the end of the ferrule - this prevents the wound cables in the housing from pulling through.
I have seen this problem with both STI and Jaqwire der cables but not with Campagnolo (although I'm sure it's happened). Most often the ferrule used was the nylon only one and this led to the failure. I have also seen metal ferrules fail this way as well but it doesn't happen very often.