What causes spinal canal narrowing please? And could this be the cause of my sacroiliac area  back pain as well as burning in cervical area.

Many possibilities. There are several causes for si joint pain but spinal canal narrowing is not one of them. Narrowing of the spinal canal in the lumbar spine does not cause neck pain. In short, none of your symptoms seem to be due to canal narrowing. This doesn't mean they can't be treated. Both of these symptoms are eminately treatable.
Several things. The spinal canal is made up of small bones held together by ligaments and joints and separated by discs. Any combination of disc bulging, ligamet thickening, bone spurs and arthritis of the joints can lead to narrowing/spinal stenosis. Anterior or posterior slipping of individual spinal componants can also cause local narrowig. Congenital narrowing can also occur. Pain or burning may result.
No and maybe. Spinal canal narrowing is what we call central canal stenosis. This can certainly cause burning in the cervical spine area if the stenosis is in that area. Spinal canal stenosis does not cause sacroiliac joint pain, but it can cause low back pain (usually when walking). Sacroiliac area pain is normally caused by a problem with the sacroiliac joint itself.