On rare occasions, a router can jam up. Your minecraft client will send a ping to the server if you are logging off, telling the server that you are disconecting. If you log off right at the time that your router jams, your computer can't tell the server that you are leaving, therefore, the server thinks that you are offline. Eventually (60 seconds by default) the server will realize that you are no longer online (as long as you don't try to log on during those 60 seconds) and the server will think that your internet crashed. This is also why if you X-out of minecraft when you are on a server, you will be in the game for about a minute, or if your game crashes.