Could be a bug, in which case it might be worth notifying the devs. It could be something as simple as the game already recognizing the aggressive young trikes as herdmates, so you can't deliver or receive damage from them. Maybe, when you were accepted in the herd, one of your earlier herdmates accidentally trampled another member, leading them to go aggro, even though the game was already assuming you were accepted in their herd.

A possible sollution could be just sleeping, and see if the problem goes away (perhaps it will despawn the aggressive herdmates).