Several changes in circumstances can trigger that turtles who used to get along now fight or pester each other:
The turtles have been moved to a new environment. This is especially problematic if the new enclosure is smaller or has fewer hide places and structures.
Turtles that grew up together and got along may "discover" that they are male/female when moved into a new environment and change their behavior towards each other.
It is natural for male turtles to chase away other males and to chase females. Outdoors, a female not ready to mate will simply swim away and hide; or, on occasion the female with turn back on the male and scare him away.
Injuries do happen but they are usually not severe.