The very first wormhole "brawl" was conducted on the 11th of March, 2009, and won by Against ALL Authorities. At the moment of victory, they were awarded the Wormhole Title Belt. They then lost the belt to Empire of Serenity when they lost a brawl against them two months later. The belt is then transferred to each group who beats the group who owned it before them.
"Major fights" are categorized here as any fight where at least one side shows 10+(ish) on the BR and over a billion(ish) ISK lost, and it at least appears to not just be a gank (you can't take the belt from krabs, for example). If one side only has a few ships, but those ships are configured in a way that looks like they were intended for a fleet fight (buffer instead of active tank, presence of logi, etc) that will be considered eligible for a "fight".
"Winning" a fight is generally ISK positivity, unless there's clear historical data otherwise.
Abdication Rules: Belt owners will hold their belts for a period of 2 months after their period of significant WH activity, at which point the belt will be abdicated and passed to the winner of the next major fight.