There's at least 1 other thread on this, but I have the same problem.  In my case it isn't limited to Sedna - though it seems more common with her and Rook than the others. 

From the little experience I have with it, it seems to happen a lot in multi-player games most often, and gets more frequent in single player games the longer I play without re-starting the game (about an hour or so into a tournament, for example).  Once it happens, it stays frequent until I reboot the computer.

  Another clue is that one time when this happened, the next several games had a second glitch:  No mouse pointer showed up when the game started, but instead, every time I moved the mouse, the entire screen would rotate (kind of like i was using mouse-look or something).  Re-booting seemed to solve this one also.