Without an aggro-establishing skill like the Knight's Provoke, it was impossible to suddenly switch the target of the aggro. Even OT required that certain conditions be met. Otherwise, if it were as simple as your damage surpassed mine so the aggro immediately transferred, then two characters could simply take turns attacking and ping-pong the boss back and forth until it died.
The aggro system wasn't designed so foolishly. To cause OT, the aggro had to surpass a certain percentage. Because of this, the fact that Dancing Rain had managed to suddenly pull away the aggro meant that they had prepared for this long in advance.
Dancing Rain was leading the boss away just like this, closely followed by Steamed Bun Invasion and Deception. Yet Lin Jingyan couldn't do a thing to stop them, because Ye Xiu was right in front him.