If you grew up reading a Western language, you're used to thinking that text should move from left to right. So, as the eye moves across a line of text, it is moving from left to right.
In the above picture, the waterfall is flowing in a right to left direction, and the text is flowing up the waterfall. Since we all know that waterfalls fall down, there is a disconnect in the direction of the flow of water with the flow of the text. The viewer's eye moves in the wrong direction to read the text.