When I bend over to garden why do I feel like a rubber band is snapping in my lower back?

Tight low back. It is most likely that you have tight muscles in your lower back and perhaps tight hamstring muscles as well. Often, physical therapy can be helpful.
Is there pain. The low back has a few structures that can create a snapping sound. Assuming you do not have any history of trauma or arthritis it can be a ligament called the iliolumbar ligament that can snap over your bones. Also, if your joints are hypermobile, meaning you have more flexibility than mos, the snapping feeling/sound could be coming from your joints. If there is pain see medical attention.