Duck Running at Me With Head Down
We had 8 khakis for a while (now down to 6 adults + 1 duckling) and the only time I've ever seen the "head down, neck out" posture is when one duck is trying to protect something, be it a male protecting a female from other males or a female protecting her babies from us or a duck chasing off a chicken that's gotten too close. If the duck comes toward you with a swaying head and open bill, I'd say it was trying to run you off. Especially if there's any hissing or other distressed sounds. The wagging tail isn't like a dog's -- it doesn't necessarily indicate friendliness or happiness. Sometimes the ducks shake their tails when they're agitated. So it may be that this duck is trying to scare you off but isn't aggressive enough to snap/nip at you when you pet it. Next time it happens, try to notice whether the bird's mate is nearby b/c it may just be trying to protect her.