None of the dogs I've seen or heard of the behavior existing in have been hit. In fact, the dogs do not exhibit a fear response to noise aversive or collar corrections and are generally confident in their behaviors, and none of the dogs had been corrected (in any way shape or form) during the training sessions where they exhibit the odd behavior.
2 of the dogs are Border Collies, 1 is an Australian Shepherd.
Behavior typically starts suddenly in the middle of a training session and in some cases will progressively worsen until the dog becomes fearful of any movement on the trainers part.
Things that have been ruled out: tooth sensitivities, static shock, accidental pairing of treat with a collar correction, stepping on dog accidentally while giving the treat.
Here's the short video of a heeling session:
The dog in this video only displays the behavior while taking a treat in motion (while heeling or while treating during a walk to distract/redirect)
He has had his teeth examined by a vet and no problems were noted. His eye sight is good (and he has been examined by an ophthalmologist), he is wearing a padded harness to prevent any kind of uncomfortable tension.
The other two dogs will display a similar behavior suddenly when being treated in a stationary position. In fact, they are LESS willing to continue working (the dog in the video wants to work, but is reacting while being treated) It should also be noted that even though the dog in the video is working, he is showing signs of stress which occurred only after the fear/pain response started... the training continues for the purpose of getting a video. The other dogs will shut down completely and becoming increasingly fearful to the point where when the owner dropped the leash the dog panicked.
Ok community, I now implore you to explain the behavior. I am at a loss.
ETA: none of the actions on the part of the trainer (the treat giving, the bending over, the walking, the standing still, etc) seem to have an effect on the dog in and of itself. I know, in the video, it looks as though the bending over causes the reaction at one point, but the dog is fine with people bending over and standing up right next to it (and over top of it). All of the dogs will eat treats readily and have no sensitivities. Handing the treat to the dog will not always warrant this response. And typically, the dog in the video walks a lot closer while out on a walk!