One of the things I've been wondering about is whether this brisket was unusually lean. Maybe that's why it cooked so fast.
BTW, 3AM it was down to 150 so I put it back in the pit ( I was cooking ribs) and it went back up to 185.
Good that it worked out okay. 150 is an okay temp--it might have well been the return to the pit that made it a little dry. It's hard to tell with leaner meats.
What happens when one continues to apply heat during a roast's resting phase is that it continues to cook. Normally, when meat rests, the juices that were forced into the center of the roast are able to migrate back toward the edges as the proteins reconfigure during resting.
When heat is applied during the rest this process can be altered or arrested, depending on how high the applied heat is and what the temp of the roast is when pulled from the cooker. If the heat is high enough the roast just continues to cook. At a lower temp of applied heat the protein reconfig thing happens but, as heat continues, the proteins lose their ability to maintain this structure and juices leak out of the meat (the same thing that happens in overcooking). This happens first at the edges of the roast then works its way in. When (and how much) this will occur is a crap shoot; it depends on several factors but the line is much finer with already lean meats. You can extend the amount of time you can rest the meat with applied heat if the heat is lower than the temp of the roast when pulled (but above the 'danger zone' of 140, of course). This is an alternative if faced with having to hold meat for an extended period; or pull and rest 20-30 min uncovered, cool quickly in the fridge, wrap, and reheat later. (For brisket, turkey breast, and other leaner meats, I find it better to reheat whole.)