This is the kind of posture you are talking about, right (the guy finishing his tsuki in front). With the back hip sticking out . I think it actually comes from the step wise way of learning Iwama Aikido. Because there are so many pauses in between the movements, there is a tendency to lean into the back hip. When I watch Saito Sensei do Aikido, it's not very pronounced, however many of his students do it a lot. It's like the bokken bounce at the end of the suburi, Saito sensei's is noticeable, but his student's is much more so.