And sometimes a founder moves on (either in game, or out of the game). Or wants to share the responsibility. And it's not fair to the new founder that they don't have all the same rights the original founder did - after all at some point the state of the guild may be more of their making than said original founder. Yes there's risks when you share founder duties - but sometimes those risks are worth it.
Particularly these days, the consequences for losing "your" guild are relatively low - some levels that are almost inconsequential, and the treasury, which might be more consequential - but is the product of more than just you, and still very replaceable. If people want to follow you still, create a new guild and it'll be up and running in no time.
If anything FoE is already very founder-centric where if you stay active and don't promote another founder, the guild is always yours. There's other games where the membership can stage a coup, calling for a vote on leadership and forcing an existing founder out - in FoE that only works on a voluntary basis with the cooperation of an existing founder

Most splits come about by people leaving the guild and taking their friends with them to another guild(new or existing) that they believe will run the way a guild "should".
The only way an additional founder that can unseat you will be promoted without your say-so is if you're inactive for a prolonged period - this exists because sometimes founders disappear never to return, and it'd be a shame if an otherwise functional guild became defunct because of that. So if that happens another member will be automatically promoted. You can avoid this by not having guildmates or by not disappearing for long periods of time