Another Reason
If it is true that there was already tension between the four, it did not help that the Queen chose Harry to be the Youth Ambassador to the Commonwealth. Reports claim that it hurt William to be overlooked for this position. That said, he had the power to veto the appointment but did not intervene.
Confirmation
A lot of royal fanatics would like to brush off the thought of conflict and blame the press for stirring up drama. However, there was a time when the Fab Four were asked if they ever get into disagreements. William was the one to break the awkward silence by firmly saying, “Oh yes.”