It's a classical optical illusion named after its discoverer, German astrophysicist Johann Karl Friedrich Zöllner.
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 fool your brain that the impression that one end of the longer lines is nearer to us 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.
Scientists hypothesize that the effect you are seeing here is caused by what they call acute-angle expansion. Acute-angle expansion means that the acute (< 90 deg.) angles created by the intersection of the diagonal lines with the center line appear to be wider than they actually are. Since the diagonal lines are all grouped together (and are already tilted anyway), the center line appears to tilt away from the diagonal lines, since each segment of the line is the leg of an "expanded" acute angle.
When you see this image it looks like it is all unparallel to each other but the lines are just some lines are longer and shorter than the other.
"Zöllner Illusion." Wikipedia. Wikimedia Foundation, n.d. Web. 17 Mar. 2015.
"Zöllner Illusion." Zöllner Illusion. N.p., n.d. Web. 18 Mar. 2015.