Dogs are territorial animals. They are comfortable around known smell and people. Crowded places like Dog Shows have so many unfamiliar smell of people and dogs unknown to them. Hence their behaviour changes when they are in such a confused territory. They can be very aggressive or very quite depending upon their nature.
The best thing to do is to start training them atleast 2 months in advance before you take them to a dog show. Initiate them to stay in crowded places by taking them to places like Bus-stands, crowded market etc. Repeat this for as many times as it takes for them to get used to it.