I would think of it as a life history strategy to deal with hostile conditions; they fall, get crushed, broken up and beat up. Low and behold, more polyps emerge. Fungia (and other plates) are successful due to their ability to do just what you have documented and are well known for it. In your tank, it may have been that conditions were so good that the best strategy was to divide up and exploit the resources. just guessing...
Why individual polyps may be larger than others? Initial conditions (e.g. more underlying tissue) may have been different giving some a head start, of course imperceptible to you and me. Another factor may be the slight differences in microhabitat conditions leading to faster growth in some.