Sometimes, it's a bad game. Texas Tech quarterback Graham Harrell was a front-runner late in 2008 until the Red Raiders were blown out 65-21 at Oklahoma.
It might be an off-field incident. Florida State's Peter Warrick was a leading candidate in 1999, then was caught buying clothes at a Tallahassee store at ridiculously discounted prices. A two-game suspension and the negative publicity torpedoed his chances.
Perhaps another player wrecks a bid. That was the case in 2009. Texas quarterback Colt McCoy just needed to avoid a disaster in the Big 12 championship game to get the trophy. But he could not avoid Nebraska defensive tackle Ndamukong Suh - i.e., the disaster. Suh led a ferocious pass rush that sacked McCoy nine times and harassed him into throwing three interceptions. Even though Texas won on a last-play field goal, McCoy's chances of winning the Heisman also were sacked.
And there is injury. In 2007, quarterback Dennis Dixon appeared on the way to becoming the first Oregon player to win the trophy until his knee buckled in the first half against Arizona in the 10th game of the season.
Oregon fans are hopeful that's not happening again.
Ducks tailback LaMichael James entered the season as a strong Heisman contender, but his campaign was dealt a setback when he managed just 54 rushing yards in a season-opening loss to LSU.
He stormed back into contention by exceeding 200 rushing yards in three consecutive games. He was the first player to rush for at least 200 yards in three consecutive games since Tulane's Matt Forte in 2007. But James dislocated his left elbow against California on Thursday, and his Heisman campaign could be derailed if he misses an extended amount of time.
James insisted that won't be the case.
"It's not a season-ending injury," he said. "I'll be fine. I'm tough. I'm a warrior and I'm never going to quit on my team."
At the halfway point of the regular season, there are at least a dozen players with realistic Heisman hopes.
Oklahoma quarterback Landry Jones boosted his candidacy with a big performance in a 55-17 victory over archrival Texas. He passed for 367 yards and three touchdowns.
"In the second quarter he caught fire," Texas defensive coordinator Manny Diaz said. "We knew that the whole key would be getting him off his rhythm, and for about 20 minutes there, we did not get him off his rhythm and he made us pay."
The top five Heisman candidates this week:
1. RB Trent Richardson, Alabama: Last week's leader rushed for 107 yards and a touchdown in a 34-0 victory over Vanderbilt. He has rushed for 729 yards and 11 touchdowns despite sharing carries and facing three teams ranked among the nation's top 32 in rushing defense.
2. QB Andrew Luck, Stanford: He completed nearly 79 percent of his passes in a 48-7 victory over Colorado. Luck is third in the nation in passing efficiency. He has completed 73 percent of his attempts for 1,383 yards and 14 touchdowns with two interceptions.
3. QB Brandon Weeden, Oklahoma State: Yes, the Cowboys were facing Kansas' dreadful defense, but five touchdown passes in less than a half in a 70-28 victory still demands respect. Weeden has passed for 1,880 yards and 15 touchdowns with six interceptions.
4. QB Russell Wilson, Wisconsin: The Badgers were idle last week, but there's no forgetting Wilson's impressive performance against Nebraska. He leads the nation in passing efficiency and has thrown for 1,391 yards and 13 touchdowns with just one interception.
5. QB Landry Jones, Oklahoma: After his impressive showing against Texas, Jones has thrown for 1,814 yards and 13 touchdowns with five interceptions.