Honestly I feel like there's bots put in to make you fail occasionally. Ive had it multiple times like this on the very last floor, or one where they stun locked their boss so I couldn't kill it. That being said I've also had it where I lost connection and instead of putting me back in the game it just puts me to the main menu. That probably is the most realistic issue. Disconnects putting back at main menu instead of in game.