Real life happens, and as unreasonable as it seems, it even happens when you're gaming. The phone rings, a parent needs you to take the garbage out, a child gets out of bed for a drink of water, the cat wants out and in and out again, the spouse needs help carrying something in. That kind of thing takes precedence over a game -- you all (should) know that. When it happens, you afk to take care of it.
Chances are that this has happened to you, and you returned from afk to find that your group has raced ahead without waiting, leaving you to catch up as best you can.