Why did the Guardian Lords not submit to Yang Xiao?

I was under the impression that like the Dog Beating skill, QKDNY was supposed to be taught to by only the leader. Yet despite Yang Xiao learning it, and learning it directly from Yang Din Tian, the 4 Guardian Lords refused to accept YX as the leader after YDT's demise.

Also, why did YDT teach YX QKDNY? Did YDT meant for YX to be the leader? If so, why didn't he make it more obvious? And what was with the letter citing for Xie Xun to be the caretaker leader?