I have no idea...other than Basil getting up there somehow.

He's the one I caught up there last week, but I had our laundry hampers in front of the bookcase, so they had easier access. I took them away as soon as I saw him up there.

That book case is at least 5.5 feet tall. I guess they can jump up, but I have stuff next to the cage, so there's nowhere for them to "land". Maybe that's why the cage fell.