I actually noticed this a few times myself. Next flight it worked ok though. Then again, stuff I type in the text buffer comes out backwards at times too.
Example, I type "like this?" and it is displayed "?siht ekil"
I figured out how it happens though. If I open message window and then hold my throttle hat switch to look left and begin typing it comes out backwards.
To my knowledge I have nothing mapped to the mouse.
Maybe I spend too much time in the dipsy-doodles.