I posted about this yesterday in some random thread. Either the option or something in the cut-scene telling me that the goblins are hostile towards me. It would be enough, in my opinion, to have the cut-scene start with a goblin spotting us and attacking on site, then showing the group continuing towards the sound of an ongoing battle. Another option would be dialogue interaction between you and a grove guard, getting a request for help by the guard and stating your intentions to 1) help grove, 2) help attackers, or 3) to stay out of it.
However, it doesn't make much sense to come across the battle and have predetermined allies/enemies before either party even spot you.
