It's a phenomenon known as "Stainsor's Wall." Named after the Scirra Forum user "stainsor" (profile link here), no one is certain exactly what the origin of this strange contraption is. All we know for sure is that it works... the physics behind such a thing is beyond any human understanding.
Here is an example of Stainsor's Wall in action:
http://dl.getdropbox.com/u/529356/stain ... 20wall.cap
If one were to try and define Stainsor's Wall (and believe me, many have), you might say it's "any Solid object placed beyond the boundaries of the Layout so as to contain the movable objects within." Suffice it to say it is truly one of nature's greatest mysteries.
The earliest known manifestation of Stainsor's Wall was documented by the historian Herodotus in the 5th century BC. While taking account of the annual flooding of the Nile river, he wrote:
"Circumstances rule men; men do not rule circumstances. The rising water held at bay not by the workings of mankind, but Providence, for it hath afforded a force beyond reckoning, in the form of a wall; beneficent, yet fraught with dire consequence, for yea though it hold back the raging flood man cannot hope to recompense nor even glean it's true nature. And it is in this that man sees his helplessness, and despairs."