According to the in-game encyclopedia of Vandal Hearts: Flames of Judgment, Avery Fields is the place where "two neighboring farmers, one Urdain, the other Balastradian, feuded over the fruits of a nearby tree. The feud ended with the Balastradian farmer killed, and the Urdain driven off his land. From there, the conflict escalated until both kingdoms charged headlong into war." The encyclopedia also tells us about the War of Avery Fields as a "twenty-year war between the kingdoms of Balastrade and Urdu. Said to have escalated from a simple feud between two neighbouring farmers. Ended catastrophically at the Battle of Timion Vale, where both nations' armies were all but wiped out with no winner."
In the end, the war ended because of the fallen of the Flames of Judgment.
It's later stated in the game that the little feud between both farmers was just an excuse to start a war over the fields, since those were years of great lack of food and resources.