The spinal column has three sections that create three natural curves in your back at the neck, rib cage, and lower back.
There are inward curves at the neck and lower back (lordosis) and an outward curve at the chest level (kyphosis). In the illustration below you will notice the lower spine has a small forward curve. The lordotic curve of the lumbar spine is responsible for correct movement of the buttocks and hips.
Without this curve, your gluteal muscles are unable to contract fully.
Internal rotation and improper hinging at the pelvis when you bend work against the natural curve of your spine, this in turn will throw your back out of alignment.
Flattening your lower back by tipping your pelvis forward is harmful for the stability of your spine. If you flatten the lordotic curve of the lower back, known as hypolordosis, your glutes cannot be activated fully, and your lower back has to absorb more pressure and weight than it is designed to do, this will ultimately result in back pain.