It's hard to see exactly what's happening from the still image but the airbrush responds to tilt by spraying further along the canvas in a conical shape. Turning off Autflow prevents build up as you move along the path which can smooth out the result. If you use the Stylus Properties panel you can turn off Tilt Angle response to Tilt in the stylus if you prefer the stroke to remain circular.
Matt
ArtRage UI
Ambient Design.