How does Monster Aggro work in Genshin Impact's Co-op Mode?

I often play Co-op mode to clear domains, and I notice that I get targeted a lot even if I’m not the closest to the monster.

For example, yesterday I was running with Klee (me), Diluc, Keqing and Qiqi in the Level 80 Cecilia Garden Domain.
On Wave 2, two Hydro Abyss Mages spawned on both sides of the Arena. Diluc and me were hitting the closer one, and Keqing and Qiqi were hitting the further one. While I was concentrating on dodging the attacks of the closer Abyss Mage, suddenly I got hit by the further one halfway across the Arena which took out 50% of my 12,000 HP.

Why did the further Abyss Mage target me, instead of the closer Keqing and QiQi? How does the aggro work?