What does it mean when pitch numbers move all over the HUD screen?
Updated answer based on F-16 specifics.

The top arrow shows the nose position—pitch angle, in the F-16 it is based on the Gun Cross (Boresight Cross), which also indicates the fuselage reference line.

The middle arrow shows the position of the flight path marker (FPM, also known as FPV), it shows where the aircraft is going. The vertical distance (angle) between both symbols is the angle of attack.

The bottom arrow is an air-to-air attack mode called a missile diamond. Don't focus on it.



The FPM can go sideways in crosswind and/or with rudder input, as the plane would be slipping, skidding, or flying sideways. That's the horizontal distance (angle).

The pitch angle numbers/lines—scale—moves with the FPM centered on it, unless the pilot wants it to be centered around the nose. The latter is called caged mode.

The rotation of the scale indicates the bank angle.

Now that we know all this, a direct answer is: the plane's nose relative to where it was going was all over the place, and so was the pitch scale.