From what I remember its a random spawn to be fair for all and keep things from being locked down like they where.
IE you kill HoH raid bosses you and your guild are locked out for 8 hrs. However the respawn on that raid boss is like 4-6 (iirc?) that way it's up for others to try for sometime before reopening to the last guild that killed it.
Not sure I agree with a timer being added saying when it would repop because then there's the chance that many will be there at the time it pops or just before all wanting to kill as they where given a time of respawn.
Whats wrong with random spawns and waiting/checking for repop? I mean it's only been that way for 8 years?
I personally love the way HoH is done with raid boss spawn timers, it's the only zone of its kind and stays very populated daily. HoH raid names being killed daily and more then once a day, so many farm these events still.