How Does Poor Posture Occur?
Poor posture can occur under any number of reasons, but what those reasons all have in common are our bones, and how our muscles develop and adapt. Our bodies are automatic adjusters, and we use something that professionals refer to as the 'Righting Reflex'. This reflex stems from our Vestibular System, a sensory system that provides the main contribution to our sense of balance and spatial orientation. The Vestibular system is located deep within our ears and it is a significant factor in how our body reacts to the world around us. So, for example when a person moves their head forward while walking, the rest of their body will adjust into a more comfortable position which can lead to slouching. This is why it is recommended to keep you chin at a 90 degree angle with the ground, as to keep your form straight.
Where everything falls out of wack is when we continuously put ourselves in a poor form, our muscles will adjust to that position and over time our bodies will remain in its adapted state. This can occur with both static posture (sitting for example) and dynamic posture (exercising). To note, a single occurance of being in an misaligned position will not harm us, however if a person is frequent with form development, their bodies will eventually adapt.
Now, when we take a look at dynamic posture our bodies are in a constant 'action and response' state. Meaning that as we move, our bodies will counteract with this movement to adjust for balance. When we grow fatigued from exercise our bodies will maintain its righting reflex function, and if the muscles are not strong enough, our bigger muscles will begin to rely on smaller muscles to help (like when your bag is too heavy so your body begins to slouch as an adjustment), only causing more strain as smaller muscles are not designed to carry weight. The exercise exertion will lead to poor form as your body is trying to adjust, leading to rolling shoulders, misaligned spine, forward head etc. And over time, if someone is continuously put into an misaligned position, their bodies will begin to adjust to a new normal state.
(some very simple notebook doodles to help iterate my point)
This information was explained to me in an interview with a physiotherapist