I think he gassed out. It's telling when a guy close to 20 years older than you is out working late in a fight. Like other posters have stated he needs to trim down to improve his cardio. He has enough butter on him to easily make 200 pounds. I suspect he works out hard but his diet in between fights is atrocious and he does little exercise. As Wlad has shown you need to exercise when you have down time if you want to keep your physique tight for a future bout.
Ps. Mansour who is about the age of Briggs shows that you can take on younger live opponents at 40+, but Briggs the blow hard would probably want no business with either fighter that fought tonight. If Wlad were to take on a senior citizen let it be Mansour or Tarver as they at least have beaten ESPN level fighters.
I agree, Joey difinately showed signs of fatigue. He just started stepping to the right, switching to south-paw, but did not throw any significant punches. In other words, he was posing for the second half of the fight.
Briggs has a history of gassing as well. I think there was some issues with asthma, or steroids, or both.
In any case, Briggs would get dismantled and Tarver, due to his size would get knocked-out early if either gets in the ring with Wlad.