Front line/rear guard

Just how exactly is determined who's front line and who's rear guard? And is it possible to leave front line for rear guard to make space for others? I know you can jump into front line if there's space, but I've in the most recent GvG found my character jumping back to rear guard seemingly randomly, is this a bug or a feature?

You can actually find this information in-game under Menu > Help > GvG.

The position of front line is set to be the 5 strongest members of the guild. While it is possible for back line players to jump to the front via Leap of Faith, there is currently no way for front line to move to the back line.

Oh, yes, I found it. It was hidden at the end of the paragraph after I'd stopped reading.

So what happened then that my character was moved back to rear guard twice during the last battle?

Thanks for the answer!