With 4:40 to go in the 4th quarter, facing a 4th and 1 from their own 39 and trailing by 4, Stanford elected to punt. Never mind that the Cardinal were averaging 5.6 YPC on the game and had one of the best players in the country in Christian McCaffrey in the backfield. David Shaw made a bad decision.
There’s an obvious reason you’d punt in that situation: you trust your defense to hold and get the ball back. It obviously helps if your opponent has a track record of playing conservative football with a lead left.
In short, this only works when you’re playing a coach like Jim Mora.
Coach Mora had been showing signs of being more aggressive. He’s attempted to convert on 4th down more this year than he has in the past, especially in non-late game situations (i.e. not losing in the 4th quarter). The defense has made a more concerted effort to apply pressure with blitzing linebackers and safeties, a departure from previous seasons. In short, Mora was trying to prove he wanted to win games.
But Shaw called Mora’s bluff and UCLA paid the price.
The problem, of course, was that Mora revealed his tell. With 7:09 left in the 4th, UCLA had a 3rd and 2 from the Stanford 18, and UCLA ran forward for no gain. The Bruins were faced with the option of kicking a field goal and give the team a 4 point lead or attempt to convert a first down and extend a drive that had lasted 3:46 and gone for 57 yards at that point. In short, play conservatively and put points on the board, or try and play for the win.
Mora chose the former and Shaw had seen his tell.
From that point on, the decision to punt becomes inevitable. UCLA was not going to do anything beyond run and throw safe, short passes once they were given the ball and, defensively, they were going to use the same prevent defense they had employed in the last BYU scoring drive from the previous game - keep everything in front and not commit the same pressure that had existed up to that point.
David Shaw made a bad football move, but he was rewarded because he called Jim Mora’s bluff, and that's why UCLA lost the game.