This is an illusion that makes your brain think that these lines are slanted. The offset blocks trick your mind into seeing slanted layers that are not really there.
This illusion is called a geometric-optical illusion, which means that are brain is trying to find orderly images from a distorted image. The broken black and white squares are on top of each other mess with your visual system, and that causes the "slanted lines" to appear. If the squares were on top of each other, the lines would appear to be straight.
"Horizontal Lines." Optics For Kids. Aaron Lee Cecala, n.d. Web. 16 Mar. 2015.
"Geometric Illusions." Geometric Illusions. N.p., n.d. Web. 17 Mar. 2015.