At the beginning of a game, the mouse pointer is positioned on the lower right end of the screen, making the game scroll immediately when the game starts so that the view looses focus on your units. This is especially irritating for new players, since it can happen that the view would scroll to a place where a different player's units are, possibly making you think those would be your units (but then you can't control them).
This happens since one of the latest betas, it did not happen before. I usually use 800x600 resolution on my Linux system.
