Adoption events are stressful for potential adopters too. They go to these events, meet many, many different animals all needing a home, and are often trying to decided in the space of minutes or hours whether a dog or cat will be the right fit for the rest of their life. It's especially hard to do considering that the animals at the adoption event are generally more stressed than anyone else, so it's hard to get an idea of what they are truly like.
But adoption events are the worst for the foster pets. They're taken to a strange place surrounded by strange dogs or cats, and are constantly bombarded by the noise and excitement going on around them. People are constantly coming and going, stopping to pet and greet the animals. Strange dogs are barking or coming up to greet them as well. Often the foster animals are placed into an unfamiliar crate and left alone while their foster parent goes off to talk to a potential adopter or take care of other chores. It's understandable that so many pictures of animals taken at adoption events show them looking nervous or fearful.
To give you a small idea of what it's like, please watch the following video. This is Ziggy in his crate during a recent adoption event. Although he can't hear the noise going on around him, the fact that he is deaf makes him more vulnerable to someone coming up behind him and surprising him. Also, this is in the middle of the day so the stress has been mounting, and he's been placed into a crate while I go off and take pictures of other pets. When I return to him, I open the crate door and put the camera in his face to take a video to show people how stressed he gets.
On second thought, this may not be such a good example after all. Let me change the above statement. For some pets, adoption events are not at all stressful. They find a place to relax, find a good chewy, and close their eyes to block out the world around them as they focus their full attention on the joy of chewies. They easily ignore all sounds, all other pets, and even the camera in their face as they chew. Adoption events are some of their favorite places to go!