I Had A Burst Fracture At L4 As A Result

I had a burst fracture at L4 as a result of a fall. I had a spinal fusion from L2 to S1. Because of severe pain after 16 months, I had another surgery to remove all hardware. Had a small fracture at L3 one year later. I had a lumbar MRI last week and the report mentions minimal grade 1 degenerative spondylolisthesis at L4 upon L5. When my hardware was removed I was told that there was fusion from L4 to S1 so why would there be slippage? S