While dyslexia is actually a cluster of related issues, a common one seems to be with dimensionality. Basically, the reader’s brain assumes the objects are 3 dimensional. When the eyes make micro adjustments, the letters don’t rotate, since they are 2D. The brain misinterprets this as them rotating, or moving. This is perceived as them flickering or moving, in the corner of your eye.

There are several ways to break this effect. I suspect the shape is intended to mess with and slightly overload the depth sense. Strong colours can also disrupt it. E.g. via a coloured filter or glasses.