It was simple.
The Seahawks called a timeout. After the timeout they had too many players on the field and tried to call another timeout (which they can’t do). They should have been penalized, but the refs instead just stop play and did nothing.
The penalty would have given the Cowboys a 1st down and opportunity for touchdown. They had to settle for a field goal.
The refs admitted their mistake.
“It’s double jeopardy,” referee Carl Cheffers said in a pool report. “The timeout should not have been granted. Obviously, the snap did not happen. At the time, the timeout was erroneously granted. It was not, by rule, 12 men on the field. We just made a mistake.”
If officials would have flagged Seattle for 12 men on the field, the Cowboys would have gotten a first down at the 12.
It wasn’t like the Miami-Duke game, but still a major screwup by refs.