The reason is, lets say A is Main case which has sub-cases B,C and D. In this for A, I have added B,C and D as child cases while creating. And from each sub-case B,C and D added Main case as child case under Child case types of pyDefault individually . This formed a loop, which Pega is unable to display on case explorer.
After removing A under Child case Types on each sub-case B,C and D. The case-explorer displayed properly.
As per my understanding initially you added B, C and D as child cases in A, later you added A as child case in B, C and D individually.
If this is what you had done then i don't think its the right approach as you are forming the infinite loop between A and B,C,D. Because in first case you made B, C and D to be childs of A then in the second case you are trying to make A as child to B, C and D.