I never figured it out.
He was never a bar climber & his cage wasn’t tall but I couldn’t totally rule bar climbing & possibly falling as a cause even though it seemed really unlikely. There was nothing in the cage to fall off or gaps to get stuck in so an injury of some kind would have been a bit strange.
I saw him in the morning & he was absolutely fine then when he came out later in the day he was just dragging his back legs with no use in them at all so I guess some kind of inflammation had set in somewhere, most likely involving his spine.
I took him straight to the vet & he couldn’t find any sign of anything wrong either.
He was on metacam for about a week, still no change & then one day after 10 days or maybe slightly longer I heard a noise, went to the cage to look & there he was running in his wheel perfectly fine!
It’s still a bit of a mystery.