Let's see, there must have been some amount of LOX and LH2 left in the SIVB when it was abandoned. One side was burning, the other freezing, The LOX and LH2 would have completely vaporized (unevenly) after a while, which meant the inside might have been shaking it a bit and sent it tumbling (as it seems to rotate every one or two minutes) and the momentum remained ever since.
I wonder what magnitude it will reach when it will get closest to Earth, before leaving us for another number of years, or forever.
Hopefully, astronomy magazines will be full of articles about this, perhaps interviews of Bean and Gordon.

Raoul