From my personal experience, low sitting boobies are in-fact sagging boobies. With that said, you only see this occurrence in women with huge boobies because gravity pulls hard at them over time and therefore stretch ligaments resulting in them hanging lower.
It also portrays a more "natural" look to the eyes versus perky bolt-ons.
Bearing children has a significant effect too.
One thing I'd like to collect some more data points on is the effect of not wearing a bra for extended periods of time. My N=2 here- one of them, a woman in her 40s ditched the bra as soon as she got home (or if she was wearing something suitable at work, and spent all weekend hanging free. She had the most magnificent breasts, about a UK D cup and a classic busty hang to them, but without any sag at all. She reckoned that not wearing a bra helped maintain the supportive tissues and she had gone without for many years, and after a few bedroom sessions I had to agree.
The other datapoint was a slim busty young woman who often collected parts from my workshop, she always wore a loose blouse and loose jeans, no bra. Again, a D cup, firm and heavy, she only wore a bra if her mum was visiting.