The maximum at "A" exists because the rocket
starts off with a large amount of mass compared to the thrust and becasue the
launch is not perfectly vertical.
If you consider the weight of the rocket as
acting only downwards, ie no sideways component of force and effectively from the
centre of gravity, and the thrust as providing a force that is aimed towards the
centre of the rocket then, in a case where the rocket is not perfectly vertical,
there is a slight sideways force, equval to the sideways force component of
Early on in the flight, the weight of the rocket and the water in it
counter almost entirely the thrust of the rocket and the rocket does not move very much.
This allows the rocket to tilt over during the thrust phase, producing a good angle of
elevation for flight over a long horizontal distance. The screen shot on the right shows
the first second of flight and you can see how the rocket tilts over. It is produced by
making the 3D graph and then selecting [A]dopt and clicking on the values suggested.