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.