For me, I think it might have been the resemblance to quilt blocks. After I got my BFA, I didn't want to paint for 3 years and instead I worked on crazy quilt blocks. When I started drawing again it was manadala, while round were suited to a square peice of paper.
Now I am work on a series 99 cats and a bird that I want to hang all the peices on a wall to look like a giant quilt.
Also some of my recent fantasy themed works are square because I am thinking of having buttons made from them. I figured it would make a better composition if I worked from squares to begin with.