Most days the pigs will circle around the food like a pack of wolves during feeding, but sometimes when dropping food between them, it turns into a fight, and every once in a while a collision. This wouldn't happen if they would line up and leave room to put a little food in front of each one of them.
In real life it's less of a "donk" and more of a "smack," since they usually turn into each other and smack their faces sidelong together.