When my eyes follow the raindrops it's impossible to see the Chinese characters. I have really to force myself to stare at one spot to see the characters. Interesting phenomena.
It's pretty simple - the symbols don't move. Their brightness changes. So if you move your view - you jump across many symbols and obviously can't "follow" anything.
I think what apunic is noting is our natural tendency to track movement. If you try to read something too close to the leading bright letter, your eyes get hooked by it and get dragged along for the ride. You have to stay a safe distance back to be able to read anything.