In this figure the black lines seem to be unparallel, but in reality they are parallel. The shorter lines are on an angle to the longer lines. This angle helps to create the impression that one end of the longer lines is nearer to the viewer than the other end. This is very similar to the way the Wundt illusion appears. It may be that the Zöllner illusion is caused by this impression of depth.
If the illusion is printed in green on a red background and the red and green are equally bright, the illusion disappears.