An alternative idea to Keldeo & Thundurus that is less dark can be the human war was already in progress (not caused by Thundurus) and Thundurus found it annoying so sent a thunderbolt down to stop the Flames of War from consuming the region. The fire was unfortunately near where Keledeo was and resulted in her getting separated from her family but this did force the two human sides to work together and allowed peace talks to happen thus putting an end to the war.