I'm with the live ball Unfair Acts people on this one. The crowd clearly enters the field of play while the ball is still live, and could arguably have affected the options that BYU might have had.
Coincidentally, I've been in a similar situation, working LJ in the late '80s. Queensland at home against Western Australia in a interstate game. WA trailing by two, with 0:02 on the clock in the 4th quarter, goes for a long range field goal. Kick is well short, and does not even make the end zone. While the kick is in the air, and it is clear that it is well short, Queensland bench and supporters enter the field to celebrate. Ball is still live, but I flag for UC. Time had expired, so we get an untimed down after the 15 yard penalty. WA kicks again and wins on a successful FG.