I've had that happen to me before with two different mice. The cursor would travel on it's own and open stuff every 15 minutes or so. I googled for a solution and it wasn't what I expected: get rid of the mouse mat! Now I use just the desk surface and haven't had that problem since. It seems that sometimes mouse can be sensitive to any dirt or uneven surfaces even though some are supposed to work well on anything. Keep your surface clean and flat and if your mouse doesn't like it, try a sheet of paper underneath.
Sometimes it could be a stuck key or a virus though.