About:

A demotivational tunnel is a series of demotivational posters compiled either in a reply fashion, or to illustrate recursion.
(for more recursion, see: Divide By Zero, Xzibit Yo Dawg)

First one poster is made, then another poster is made using the first poster as the image in the poster, and so on.

These occur both intentionally and incidentally:

• Intentionally, when the "tunnel" effect is desired to either provide an optical illusion or to obscure the first image used.

• Incidentally, when everyone has a problem with the replies given to the poster and decide the only logical method of voicing their displeasure is by continuing the tunnel.

Commonly used in this sub-meme of demotivationals is "divide by zero," to display a continuing, endless stream of fail in the form of the demotivational in question, or to play up the optical illusion created by the borders around demotivational posters.

Uncertain Origin

Locating the very first motivational tunnel is a difficult (if not impossible) task. However, a Google search for either "motivational tunnel" or "demotivational tunnel" return results from motivatedphotos.com more than any other generator site. If this isn't the origin, it is at least the site on which they are most popular.