My theory: As soon as you arrive at the training area, the dog's anticipation and excitement instantly get cranked up a few notches. He's still in the crate, and while you're setting up, this builds. Other dogs are run, amping him up further. By now, he's probably whining. Then you opent the crate, and he wants to run out before being released, so it's "no, kennel!". Stress is building. Finally, he's out of the truck and he's rarying to go, and get's ahead of you and you correct with "heel", and running through your regimen for "heel" issues. Stress increases. You're finally in the holding blind, anticipation and stress are at 80% and the real vocalizing begins, which you correct with "sit, quiet." Guns are firing, duck calls are sounding, dogs are being sent, and Fido has to stay seated in the blind, and the stress has reached 90%, and he can't contain his vocalizing, yawning, etc. Now you walk to the line, and with more "heel" corrections, he can hardly contain himself. But instead of running a mark, you line him up for a blind. Every time you say "Heel, heel, here" while snapping or tapping your leg to get him looking in the right direction, he's whining in protest. You send him for the blind, receive the bird, line up, and send him again, then repeat. The pent-up stress has been given an outlet, he's physically winded (dogs are calmer when they're tired), and his excitement level has abated because blinds aren't as fun as marks.
That's my thinking, but like Charles C. said, I don't know, and am very interested in receiving your input.