This is another of those things that comes up *alot*, but in fairness it's pretty hard to find this with a simple search (well, it didn't appear in the first two pages when I tried ).

Shame really, because I know KC did a good write-up about it ages ago.

In short, it's because all of your characters are facing *slightly* different directions, even if you think they're not. The only time they're facing exactly the same way is after you zone in some way (instance, FP, etc).

That and the fact that not all toons are created equal - server lag and other factors will result in them facing in slightly different directions over time and you'll end up with them all over the place.

You can prove this without the mouse just by binding the same movement keys to all your characters and trying to get them to all go to the same place.

I'm not quite sure why you wouldn't want to follow the main, but I imagine it's an image thing, perhaps because you don't want the attention that we all eventually learn to live with

The closest you can get to giving the illusion of separate control is to assign character-specific movement keys so that you can move each one individually, something that's useful in certain encounters anyway.