It may be Prince William’s second turn if recent rumours are to be believed. His marriage to Princess Catherine has resulted in a family primed for reign.
However, moving on is also a part of Royal life. His father has done it (albeit under different circumstances). He may now be preparing for it too.
The mysteries of why is easily answered by what usually comes next. A Prince often wants his reign to set in properly. It may entail a new direction to do it.