The article doesn't say. In the mini-animation of when the fault occurred, after the momentary blank, you can see the bat does actually get partially concealed

The article doesn't say. In the mini-animation of when the fault occurred, after the momentary blank, you can see the bat does actually get partially concealed by something associated with the ALF sprite; it's possible that that counts as a collision. That would seem to indicate three possibilities: 1. The top of the ALF sprite is wider than it looks and has some non-transparent black in it. 2. There's another (non-transparent black) sprite there that's colliding. 3. There's a sprite multiplexing limitation that somehow counts as a collision.