If you are going to slice it and continue to let it cook over about the mid 190's it will fall apart before it slices in my opinion. If your going to pull it then take it to 205 or so. I would go to about 190 then foil and towel wrap and place it into a dry cooler to rest if it were me. Sometimes the meat just goes fast if your thermo is right and you've checked it in more than one spot and get the same reading then it should be good
according to my reading and what i've seen from folks here, briskets can do that at times. i thought that it happened at lower temperatures, but i'm pretty new to brisket so anything is possible, i suppose.