A toilet may flush itself when the Douglas valve seal becomes warped.  When this seal is warped, the toilet tank continually leaks water into the bowl, triggering the fill sensors, indicating that the bowl is filled and needs to be flushed.  It’s the same as when you fill the toilet with water and it flushes itself.

Run a check by putting several drops of food coloring into the toilet tank.  If you see food coloring in the bowl, you will know the tank is leaking and you will need to have the valve replaced.