I think we have your answer. Deflection means the floor bends. It is weaker around the toilet flange because there is a big hole cut there, and often a seam, for the same reason you placed the cement board seams there. As a result of this weaker area, the floor deflects or bends, and that is what caused the rigid tile to break.
I seriously doubt you did anything wrong in installing the floor, and I would be surprised to see anything else give. At least this one cracked tile is hidden.
This has been a good post, and makes us aware of where floors can be weaker, and the importance of avoiding intersecting seams in those areas. I would not have thought of it.