B runs in circles effect

I am wondering why B sometimes runs in circles when he moves to a block. I made a demo where it is impossible to stay on a grass block - B always runs away!

I found it only happens if there are blocks above him. Is this the "low ceiling" trick? I can't figure out why B does this, as there is plenty of room for him to stand below the blocks.

 

It could be useful to make B follow a path through a tunnel - I'm sure I've seen this on levels where he disappears into a doorway and magically comes out the other side!

Also useful if B can walk on blocks he normally can't - thanks for that trick!

 

Note it's possible to cancel the effect by adding a block at the side.

The effect still happens on the other side.

I can also stop the effect with other combinations of this level - sometimes it happens, other times not. And if I just have the grass blocks and no silver ones for B to run to, he stays on the grass.

 

Yes! B can be made to run around constantly! Just tap on a grass block: