It likely changed because one or more of your channel strips got converted to surround. This may have happened inadvertently as a result of calling up a channel strip setting containing a surround version plug in. Find the surround channel strips and switch them back to stereo. That should do the trick.
To add to this, one of the channels that often changes if you audition something in surround is the Master channel. It's easy to miss this channel strip, because it doesn't appear in the Arrange or Inspector, only in the Mixer. So if you think you've changed every track in the Arrange to stereo and it still seems Surround, be sure to change your Master channel strip back to stereo.
It looks like the channel strip of the selected track in the above picture is actually the Master, just ("real") track named to "Out 1-2". (Are you sure this is no test? ;-).
Control- or right-click the track header, in the context menu select:
Reassign Track > Mixer > Output Stereo > Output 1-2 Stereo Output.
(Don't mind if the last two items might be named a little differently - this is how the names would be in a 'virginal' Logic 9 project).