It would be convenient to picture the intra-abdomen cavity as a barrel-shaped container with nothing but air, but of course the abdomen is filled with vital organs and tissues leaving very little space for anything else. Nonetheless, and for the sake of biomechanics, let's imagine the intra-abdomen with just the skeletal frame, muscular sheaths and an air-tight skin layer.
The lid of the barrel-shaped container is the diaphragm as seen in the image, which is a very durable muscle that is constantly at work. When contracted, the diaphragmatic muscles press downwards into the center of the cavity creating a compressive force. The left and right sides of the barrel-shaped container are lined with the transversus abdominis muscles that compress the abdominal contents from two sides when contracted, further increasing the overall IAP. Next, the bottom of the container is layered by pelvic floor muscles. Its function isn't to compress into the central cavity, but to act as a solid base in maintaining IAP. Lastly, the spinal column is the pillar that holds the barrel-shaped container upright and does another very important task in acting as a shock absorber - like the suspensions in a car. Having a strong back means that these shock absorbers (termed "discs") aren't being overused or compressed beyond maximal capacity, because otherwise, the said object could experience a condition commonly known as a "compressed disc" or "slipped disc" or "disc herniation".
What causes one to have a weak back? Let me backtrack a bit. Imagine that the muscles responsible for IAP have weakened over time perhaps due to overuse, repetitive labour, postural stress or simply due to a sedentary lifestyle. The weakened structures will contribute to less intra-abdominal pressure. Mind that the weakening process can take anywhere from a couple months to decades to fully take place. As a result, there is overall less IAP support (less shock absorbing) at the spinal column. Conversely, there is an increased susceptibility of damage to these shock absorbers - technically known as the "intervertebral disc" (IVD). In the diseased state, the shock absorbers (discs) can slip around, change shape and cause a lot of pain. And in more severe cases, the lumbar spine could deform and compress onto nerves and muscles. Within this context, the medical terms: sciatica, spondylolisthesis, stenosis and intervertebral disc rupture are born out of this phenomenom of a "loss of IAP". Needless to say, there certainly are other factors that can contribute to the said diseases that has troubled so many in their lives, but the loss of biomechanical support (or IAP), to this day, stands as the sole reason for.