It wasn't one or two things. It was just about everything.
There are a variety of explanations out about what exactly happened to the Dallas Cowboys in the collapse against the Atlanta Falcons. The defense was unable to stop the Falcons to help out the offense. With Brandon Weeden at quarterback, the Cowboys hardly attempted to go deep, which could be due to Weeden, the receivers, the coaching staff, the coverage, or all of the above. After a great start, the running game went completely dead. There were questionable coaching decisions beyond the game plan for Weeden.