This is happening to me, too, on several toons. It happens when I am in combat. I think Vyr is right, when an enemy lands a CC on me. My toon will orient to be perpendicular to the ground for as long as I have my weapon drawn. If I stow the weapon, the toon re-orients to a vertical position. https://dropbox.com/s/vowcpytcd7c5gns/leaning.png?dl=0