None; just shape dynamics and smoothing. In that gif, I was just using the default "hard round" brush.
Does it happen when you use keyboard brackets? [ = smaller, ] = bigger?
Bracket shortcuts don't seem to change opacity, but using alt shortcut afterward will still change opacity.
I've tried everything I can think of to duplicate the issue, but it never happens on my system.
What operating system are you using?
Are you right clicking with the mouse, or a tablet button? If the latter, what tablet, and what driver?
What GPU are you using, and what driver version?
Have you tried disabling GPU Assistance in Preferences?
If that fixes it, try different settings for Drawing Mode.
If you still have the issue after that, can you go Help > System Info > Copy and paste to this thread?