I have this too when entering zones with phased content.
These zones are like an on-the-fly instance and what you see is basically the same that happens when you enter a WotLK instance: your main gets in and the following characters break follow.

You can find out whether this is the problem or not:
When your toons break up, fly back to where they have broken off, then go back 20 yards more. Now continue to go forward again. Your toons should stop following again more or less at the same spot they broke up before.

More info about phased content:
http://www.wowinsider.com/2008/08/25...ew-instancing/