I've been doing a ton of reading about this lately myself because one of our females was pulling a little too roughly on another girl's head when mounting her in their pond. Apparently, it can be due to so many factors: too small of a flock, very hot weather (so they might get stressed out), a change in food, an illness the other ducks detect, and on and on. For some reason unknown to us, our girl stopped that behavior one day, and all's been well. Our situation wasn't as bad as those photos look, though. I wish I could be of some help.