I had a very similar thing happen when I cooked my first brisket, was all setup for a 15 hour cook and it was probe tender after 8 hours. I think it was the fat content in my brisket. I think that when you wrap a high fat content brisket, it cooks faster. Like yours, mine was perfectly cooked though. My lesson was exactly what JL said above, every cook is different and its done when its done. Great Cook!