Why do the armies spring to attack after telling each other they would not do so?

Arthur and Mordred agreed verbally to a treaty in which Mordred would receive land in exchange for one month of peace. When the two met to sign the treaty, they each had their knights in attendance. Arthur and Mordred had both told their knights not to attack unless attacked, but an accident changed everything. One of the knights in attendance was bitten in the foot by an adder (snake), and he pulled his sword to kill it. This event was directly responsible for the carnage that followed.