Guilds were placed in brackets by their current trophy count at the time guild wars first started. Since then, they have been moving up and down the brackets after every war, depending on their placement in the bracket and the hidden algorithm that handles the movement between brackets. Devs never want to publicly confirm much about it, but basically the top 1-3 guilds in the bracket will move up, the bottom 1-3 guilds will move down.
Here’s an old post on that algorithm:
The path up the brackets is long and full of guilds that seem to have no business being in that bracket. Many are guilds who used to be active, but aren’t anymore and are on their way down the bracket ladder.
There’s a few threads around on the subject, here’s the most recent one:
What bracket are you in and how fast are you climbing? There isn’t much you can do to get higher faster unfortunately. Go for the highest score possible and hope for the best. Good luck! 