They're loud. Other planes are landing and taking off while these horses are being unloaded from the truck and then waiting to be flown.
Horses have a keen sense of hearing, like many species of animals, so they would be disturbing, the same way they're disturbing to us, but they are especially disturbing to horses, who have never been exposed to any of these sounds. That's what is so stressful about transport for horses—it's exposure to stimuli they've not encountered before, and that's what is important.
The other witnesses commented about habituation and conditioning to various stimuli that they're going to encounter during transport. These feedlot animals are not accustomed to any of that, and that's what makes it particularly stressful, especially when compared to other groups that have had that kind of conditioning.