I'm sorry, but I fail to see how Tennessee, Washington, or any "tough defense" is a "big game." It's not. Those are not top 25 teams, any of them. That Quinn put up numbers against top 25 defenses Penn State and Tennessee the past two years, yet struggled against top 25 defenses Michigan and USC, says that the pressure of the game, and not of the defense, gets to Quinn.
And YES, he struggled against SC this year. HE heaved it 45 times and completed fewer than half his passes.
And if you want an explanation of how that constitutes choking, here it is. Against USC, score 0-0, first possession-
3rd-9, USC29 12:25 B. Quinn incomplete pass to the right
4th-9, USC29 12:16 B. Quinn incomplete pass to the right
Down 7, momentum gone, second possession-
2nd-6, UND31 7:30 B. Quinn incomplete pass to the left
3rd-6, UND31 7:26 B. Quinn incomplete pass to the left
4th-6, UND31 7:23 G. Price punt. D. Reed returned punt for 42 yards
Down 14, in the Red Zone-
1st-10, USC10 0:55 B. Quinn incomplete pass to the right
2nd-10, USC10 0:49 B. Quinn incomplete pass to the left
3rd-10, USC10 0:44 B. Quinn incomplete pass down the middle
4th-10, USC10 0:39 C. Gioia kicked a 27-yard field goal
Down 21-10, his defense having gotten him a quick INT-
4th-4, USC46 4:49 B. Quinn incomplete pass to the right
Down 21-10, his defense having gotten him another-
3rd-6, USC32 2:33 B. Quinn incomplete pass to the left
4th-6, USC32 2:27 B. Quinn incomplete pass to the right
One last chance before halftime, down 11-
1st-10, UND27 0:30 B. Quinn incomplete pass to the left
2nd-10, UND27 0:12 B. Quinn passed to M. Freeman down the middle for 11 yard gain
1st-10, UND38 0:04 B. Quinn incomplete pass to the left
The half ended, SC soon went up 28-10, and the game was never in doubt. Draw your own conclusions.