I will give you a very concrete example of a challenge or problem that can arise.
When animals get on the trucks, they are entering a new environment, and it's stressful for them. It's a time when the risk of injury is greater. Once the animals are in the truck, if the transport takes a long time, they will lie down, so there is much less risk of injury. An animal lying down is less likely to fall and injure itself or other animals. In the event of shortened deadlines for transporting animals, the trucks may have to be unloaded and the animals taken to an unforeseen location, in less-than-optimal conditions. This increases the safety risks, not only for the animals, but also for the people handling them.