Secondary groups tend to break down into primary groups largely because we human beings have personal and emotional needs, not just impersonal, transactional needs.
In secondary groups, we are interacting with other people on the basis of impersonal factors like rules and tasks. These are, for example, the kinds of relationships we tend to have with others at our jobs. Our interactions with others are based on the fact that we are required to interact with them in order to do our work.
But these interactions tend not to be very emotionally fulfilling. We do not interact with these people as individuals about whom we care. We tend to need more than this to feel good. Therefore, our secondary groups tend to break down into primary groups. At school, we form cliques of friends who are much closer to one another than regular classmates are. The same thing tends to happen at work. We form these groups because we want relationships that will give us emotional support, not just relationships in which we are treated as relatively impersonal cogs in a machine.