I am unsure if this is affecting all guilds, but it is affecting at least 3 guilds that i know of. Guild renown decay is more than it should be. It is being calculated incorrectly somehow. This problem began happening after the last downtime, when the servers did not come up all day long. More details in this thread.
Another issue (?) that cropped up at the same time is guild renown decay now hits at 5pm EST as opposed to at night like it used to.