Iwama style student checking in here: Ethan's explanation is spot on.
I can say that my Sensei is very aware of this 'duck butt' and corresponding 'chicken leg' aspects of incorrect posture. We have many instructors who trained with Saito Sensei at various times during his life and like those who train with any instructor for a period of time they take away that portion of what they learned and do their best to pass it on. I can see what appears to be this 'pronounced forward lean' in some of these teachers and in some it is more so than in others. In a few it is not present at all. However I can assure you from getting a hold of all of them, they are very balanced, very solid and very smooth and quick (not that this thread implied otherwise
From these teachers who studied with Saito Senei for a few years to even decades... we still see 'variations' within Iwama style Aikido. It is simply another product of when one trained with a teacher and the emphasis they picked up on at that time. If that student was unable to return and train with Saito Sensei after a point when his form changed then they would not have the opportunity to adopt it and feel it for themselves and decide if it is better for them and their Aiki.
My Sensei is patient and works hard to correct us with protruding back side or the over extended/forward leaning aspects that people are discussing here. She insists on a very upright posture through all movements with strong hip in all Taijitsu and Bukiwaza as they are complimentary. Even though she demonstrates it... somehow every now and then we students perceive it differently or our bodies simply do not follow what our eyes and minds are trying to translate and tell it what to do in following Sensei.
Re-iterating what Ethan posted: I have found that people have their preferences and Iwama style is not the 'way' some people would learn about Aikido. It is just one more way, nothing more or less. Even within Iwama style there are 'flavors' taken from periods of time in Saito Senei's teaching. As with all Aikido, it makes it more interesting and just means taking time to study up and learn about it all and understand how these 'variations' come about.