Obviously there are going to be people with very different physiques, as well as approaching in trying to display their strong points and conceal weaknesses to the judges as well as to audiences/photographers. When I was trying to learn how to pose, I’d look at shots of Yates (one of my favorites) and not realizing how different my build was from his, I would lean back on certain poses, thinking I’d create the look of lower hanging lats in my front poses. In hindsight, it was horrible, and I’m thankful I eventually got past that!
The judges’ table is much lower than the eye level of most of the audience, as well as the level of most photographs you typically see of bodybuilders. This may affect how some competitors hit specific poses. I know that when I would hit shots facing away from the judges, I didn’t care how weird it looked to anyone backstage, so long as the judges only saw what I wanted them to.
With all that said,… leaning back when hitting any front pose, in my opinion, isn’t going to be helpful. It may just be a mistake on the athlete’s part, as there are so many little points to remember when you’re onstage at a higher level and you need every advantage you can get. In back poses, it could make sense, as you want to get those lumbar fibers to tense up, but not so much that you misangle your torso and take away from what might have been an impressive lat spread for example.