I think that in both cases, the causes are essentially the same - inexperience. American and Chinese people have very little vacation time. They're both big countries, so there's a lot to see and do domestically. It's quite normal for a European to have travelled abroad several times a year since childhood, but a very large proportion of American and Chinese tourists have little or no experience of international travel. A lack of experience and the desire to cram a lot into a short trip encourages large group tours, which are inherently more disruptive than independent travellers.
It's hard to be thoughtful and polite when you're being hustled from one Kodak moment to the next. It's hard to blend in with the locals when you're in a group of 30 foreign tourists. It's hard to pick up on the local etiquette when you've blasted through six countries in as many days; it's doubly hard when you're from an unusually extroverted culture.
Savvy Americans have started to leave the tour bus behind, learn the art of travel and buck the stereotypes. I'm sure that Chinese travellers will follow the same path.