Taz,,, my hardware has came loose twice and your rod may have came out the loop in the screw,, the cage should be between your vertebre. but the screw on the other hand has a loop on the end that the rod goes threw. my roRAB are moving around now,, they want to wait till its a year after surgery to make sure its fused to go in and pull them.
They say just like anything else,, some ppl's bodies reject it and starts trying to push it out,, thus is why it comes apart,,