Months ago, it was reported on the Bug Tracker that there's something wrong with the timers for Defile and Valkyrs on Lich King in ICC - namely, on retail they are always staggered by at least 5 seconds, but here on Warmane the 3rd Valkyr spawn is immediately followed by Defile, giving players no time for spreading out.
And yet, the bug is still here, and on the tracker it's marked as Unconfirmed... why? The retail evidence is overwhelming, and any player who's fought Lich King here can attest to the existence of the bug.
iirc the staff response was something along the lines of "changing this timer would cause a domino effect with other timers".
Paraphrasing here of course but they would rather keep this timer in particular like this for now rather than deal with the potential headache of fixing new bugs.
Right now the delay is next to non-existent, the 3rd Defile happens before all 3 Valkyrs even finished grabbing their targets. Compare it with any of the above videos, where you always have a reasonable window to spread out after Valkyrs.