Yeah, I dunno. I mean that fight is pretty heavily foreshadowed and there's already the option to hide (or retreat) and not participate. (I don't think some dialogues check for participation, something to fix up - albeit some grove people don't witness the fight and rely on iffy hearsay anyway.)
There is some story justification for the goblins to be in a mood to fight, and simply fall upon the party. Dialogue requires 2 willing parties, and if the goblins aren't interested... (DMG has some DCs for influencing NPCs, by memory, it's not possible to influence once fighting begins - which is why the level 2 Calm emotions spell exists.)
Grove defences are weak; Zevlor and Wyll man the gate but the others are driven back by initial volley of arrows. And these goblins are presumably part of the same warband that've driven Aradin's party back in skirmishes. Not so surprising they're over-confident.
