Update: I saw it again last night. The punching bag was moving, the light on the bag was moving, and the few spots of light on the wall were not moving.
Hmmm....
So I started concentrating on the distance between the punching bag and the spots of light on the wall. Despite the perception of the bag swaying, the bag never got closer to/further from the spots. The weirdest part is that my brain was able to accept all of it, despite the contradiction.
As a test, I reached out and touched the bag. I also put my hand very close without touching it. Both tests confirmed that the bag was not moving.
And lastly, I actually got the bag swaying so I could compare it truly moving to my perception of motion. They were very different. Most notably, the bag moved, but the light on it remained still.
I think the illusion comes from the fact that little else is lit but the bag. There isn't anything to use as a reference to establish that it's still. Just guessing, of course.
This sort of illusion wouldn't explain every paranormal experience, but I wouldn't be surprised to find it was the basis for many of them.
Dave