The stability of a rocket in flight is determined by the positions of its center of gravity (CG) and center of pressure (CP). Please read the CP article before reading this one.
We will first consider the rocket flight straight through the air, then with a small angle of attack.
PICTURE OF BASIC ROCKET FBD W/ CG & CP
This represents a stable rocket well into its flight - the rocket is flying straight. If a gust hits the rocket, there is a small force applied to the entire rocket. The net force F_aero is represented as a point force acting on the CP. (Since the center of pressure is the equivalent center of aerodynamic force, we can see this effect as a point force on the CP) we see that the rocket turns about the CG into the wind.
Let's consider the similar situation where the rocket is leaving the launch rail. It is guided vertically during the initial portion of its flight by the rail, and when the final rail button comes off of the rail, the rocket is free to move in 3 dimensions. If there is a relative wind, we end up with the following:
PICTURE OF BASIC ROCKET LEAVING THE RAIL W/ V_RAIL EXIT, AND V_WIND
This moment in the rocket flight is key. This is the moment when the rocket is moving the slowest, and the least stable point in the ascent. The angle of attack is almost always maximum at this point because the rocket is going relatively slow.
To be continued.......