It was probably because the Chinese had a 'sino-centric view of the world' so pretty much anyone who was not Chinese was sometimes automatically considered a barbarian. A LOT Europeans before the 18th century didn't use forks, (instead they cut their food with a knife then eat with their hands.) Now Chinese people have been using chopsticks for centuries, so when they saw the Europeans do that, it probably made the Chinese see Europeans as barbarians.
Because if I saw someone eating something one way that seemed less intelligent to my standards, I'd probably be like, "wait what?" XD
They also might have saw Europeans as very violent people. But honestly, the chopsticks, and how they had a sino-centric view of the world in that point of time turns a lot of people to believe that most were probably a little freaked out on the chopsticks thing. Or that the Europeans were just to violent for them.
It might've also been because that it was just to completely different cultures, and it might've been in a time of war.