I've been playing for many years and these are the reasons I see most common why guilds die:
Cliques. This thread is full of comments about cliques so let me say thing: Nobody is saying not to play with your friends, but for group content, try to go outside your circle. You're in a guild with many other people and they are not just bots that fill your online screen.
Blaming others. Mistakes happen and when they happen, they need to addressed. But throwing profanity and wishing death on someone because they forgot to dispel is 100% going to make that person leave. "Oh well I don't need them I can replace them" yeah I've seen many GM's say this only for the guild to die a few weeks or month later. You're the manager and supervisor of the guild, not a forum troll.
Drama. Keep your personal drama or guild drama to a minimum. Don't like someone? Whisper each other about it, whisper your GM or Raid leader or keep it to yourself. It's not always easy, but guild drama in guild chat or voice is just awkward and makes people not want to play with you. Imagine that you're trying to play a game of soccer but half your team is just yelling and screaming at each other for hours on end while you stand there waiting for the game to resume. Eventually you want to go home. Nobody cares about why you don't like Susan other than you. Nip that shit in the bud.
Focus your goals. Guilds with no goals just float around in game. People will come and go and very few of them will actually stick around for any amount of time. If you're a casual guild, focus on that. "We do transmog." "We run heroics and mythics on occasion." "We play music in voice chat and talk shit" Cool just have a goal, even if its a small one.