What accident triggers the battle between the two armies?
In Malory's Morte d'Arthur, the two opposing armies meet face to face to talk peace. Both armies are suspicious of the other, however, and it doesn't take much to start a fight.
An adder comes out of a bush and stings a knight in his foot. When he raises his sword to kill the snake, everyone sees it and thinks a fight is starting. Trumpets and horns blow, announcing battle, and the battle begins.
Arthur had planned to delay the fight until Lancelot could arrive with reinforcements, but the appearance of the adder takes care of that.