If you are using many addons, and in particular certain addons that accumulate a lot of data, it can make the log-in times significantly longer than normal.
Well... though that sucks, there are those of us who are willing to put up with the additional time to log on in favor of using the addons.
The problem is, sometimes this added amount of time exceeds the limit placed on how long it allows you to log-in to the game. So after waiting it ultimately decides to kick you from the game without fully logging in. Presumably had it allowed for a bit more time it would have succeeded at some point.