Wanton means a cruel and deliberate violent action. Wanton sounds a lot like "wonton". A wonton is a type of chinese dumpling. Wonton soup is a type of soup we see in Chinese cuisine. "Wanton soup attack" means that the soup attack was cruel and deliberate. It also sounds like "wonton soup". 
The Chinese chef maliciously dumped a hot broth with dumplings on an obnoxious customer . It was a wanton soup attack .