You're right, they should normally be facing the same way every time they are built - This particular problem could be down to a number of things: As Elliot mentioned, it's possible the PAK could have been moved by a teammate/enemy via vehicle (manually or bumped), a plane or other explosives could have caused it to sway, and I remember my own PAK whipping around all crazy-like when the server got a little laggy (just like AA's back in the day.
If you have any video of a soldier actually building one in reverse, post it here and I'm sure someone on the team will see it