They were watching him get beat up, of course.
Also, notice how “milkshakes” are just sarcastic accompaniments to their punching and kicking.
The assailants wore black clothing and masks, and were engaged in a counter-protest against several right-wing groups, including the Proud Boys. Ngo is a well-known chronicler of antifa activity, and has criticized their illiberal tactics on Fox News. He attended the protest in this capacity—as a journalist, covering a notable public event.
According to Ngo, his attacker stole his camera equipment. But video footage recorded by another journalist, The Oregonian‘s Jim Ryan, clearly shows an antifa activist punching Ngo in the face. Others throw milkshakes at him: